docs(appsync): fix heading levels (aws#14073)

The current heading levels for the [AppSync Docs](https://docs.aws.amazon.com/cdk/api/latest/docs/aws-appsync-readme.html) seem not aligned.

This PR tries to introduce better nesting:
* group examples under example heading
* move headings under Aurora Serverless one level up
* group types under GraphQL Types

----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

packages/@aws-cdk/aws-appsync/README.md

@@ -26,7 +26,7 @@ APIs that use GraphQL.
 
 ## Example
 
-## DynamoDB
+### DynamoDB
 
 Example of a GraphQL API with `AWS_IAM` authorization resolving into a DynamoDb
 backend data source.
@@ -95,7 +95,7 @@ demoDS.createResolver({
 });
 ```
 
-## Aurora Serverless
+### Aurora Serverless
 
 AppSync provides a data source for executing SQL commands against Amazon Aurora
 Serverless clusters. You can use AppSync resolvers to execute SQL statements
@@ -240,13 +240,13 @@ httpDs.createResolver({
 });
 ```
 
-### Schema
+## Schema
 
 Every GraphQL Api needs a schema to define the Api. CDK offers `appsync.Schema`
 for static convenience methods for various types of schema declaration: code-first
 or schema-first.
 
-#### Code-First
+### Code-First
 
 When declaring your GraphQL Api, CDK defaults to a code-first approach if the
 `schema` property is not configured.
@@ -274,7 +274,7 @@ const api = new appsync.GraphqlApi(stack, 'api', {
 
 See the [code-first schema](#Code-First-Schema) section for more details.
 
-#### Schema-First
+### Schema-First
 
 You can define your GraphQL Schema from a file on disk. For convenience, use
 the `appsync.Schema.fromAsset` to specify the file representing your schema.
@@ -286,7 +286,7 @@ const api = appsync.GraphqlApi(stack, 'api', {
 });
 ```
 
-### Imports
+## Imports
 
 Any GraphQL Api that has been created outside the stack can be imported from
 another stack into your CDK app. Utilizing the `fromXxx` function, you have
@@ -304,7 +304,7 @@ If you don't specify `graphqlArn` in `fromXxxAttributes`, CDK will autogenerate
 the expected `arn` for the imported api, given the `apiId`. For creating data
 sources and resolvers, an `apiId` is sufficient.
 
-### Permissions
+## Permissions
 
 When using `AWS_IAM` as the authorization type for GraphQL API, an IAM Role
 with correct permissions must be used for access to API.
@@ -358,7 +358,7 @@ const api = new appsync.GraphqlApi(stack, 'API', {
 api.grant(role, appsync.IamResource.custom('types/Mutation/fields/updateExample'), 'appsync:GraphQL')
 ```
 
-#### IamResource
+### IamResource
 
 In order to use the `grant` functions, you need to use the class `IamResource`.
 
@@ -368,7 +368,7 @@ In order to use the `grant` functions, you need to use the class `IamResource`.
 
 - `IamResource.all()` permits ALL resources.
 
-#### Generic Permissions
+### Generic Permissions
 
 Alternatively, you can use more generic `grant` functions to accomplish the same usage.
 
@@ -386,7 +386,7 @@ api.grantMutation(role, 'updateExample');
 api.grant(role, appsync.IamResource.ofType('Mutation', 'updateExample'), 'appsync:GraphQL');
 ```
 
-### Pipeline Resolvers and AppSync Functions
+## Pipeline Resolvers and AppSync Functions
 
 AppSync Functions are local functions that perform certain operations onto a
 backend data source. Developers can compose operations (Functions) and execute
@@ -418,7 +418,7 @@ const pipelineResolver = new appsync.Resolver(stack, 'pipeline', {
 
 Learn more about Pipeline Resolvers and AppSync Functions [here](https://docs.aws.amazon.com/appsync/latest/devguide/pipeline-resolvers.html).
 
-### Code-First Schema
+## Code-First Schema
 
 CDK offers the ability to generate your schema in a code-first approach.
 A code-first approach offers a developer workflow with:
@@ -429,7 +429,7 @@ A code-first approach offers a developer workflow with:
 
 The code-first approach allows for **dynamic** schema generation. You can generate your schema based on variables and templates to reduce code duplication.
 
-#### Code-First Example
+### Code-First Example
 
 To showcase the code-first approach. Let's try to model the following schema segment.
 
@@ -545,7 +545,7 @@ create the base Object Type (i.e. Film) and from there we can generate its respe
 
 Check out a more in-depth example [here](https://github.com/BryanPan342/starwars-code-first).
 
-#### GraphQL Types
+## GraphQL Types
 
 One of the benefits of GraphQL is its strongly typed nature. We define the
 types within an object, query, mutation, interface, etc. as **GraphQL Types**.
@@ -562,7 +562,7 @@ More concretely, GraphQL Types are simply the types appended to variables.
 Referencing the object type `Demo` in the previous example, the GraphQL Types
 is `String!` and is applied to both the names `id` and `version`.
 
-#### Directives
+### Directives
 
 `Directives` are attached to a field or type and affect the execution of queries,
 mutations, and types. With AppSync, we use `Directives` to configure authorization.
@@ -577,12 +577,12 @@ through `Cognito User Pools`
 
 To learn more about authorization and directives, read these docs [here](https://docs.aws.amazon.com/appsync/latest/devguide/security.html).
 
-#### Field and Resolvable Fields
+### Field and Resolvable Fields
 
 While `GraphqlType` is a base implementation for GraphQL fields, we have abstractions
 on top of `GraphqlType` that provide finer grain support.
 
-#### Field
+### Field
 
 `Field` extends `GraphqlType` and will allow you to define arguments. [**Interface Types**](#Interface-Types) are not resolvable and this class will allow you to define arguments,
 but not its resolvers.
@@ -609,7 +609,7 @@ const type = new appsync.InterfaceType('Node', {
 });
 ```
 
-#### Resolvable Fields
+### Resolvable Fields
 
 `ResolvableField` extends `Field` and will allow you to define arguments and its resolvers.
 [**Object Types**](#Object-Types) can have fields that resolve and perform operations on
@@ -671,7 +671,7 @@ const query = new appsync.ObjectType('Query', {
 
 Learn more about fields and resolvers [here](https://docs.aws.amazon.com/appsync/latest/devguide/resolver-mapping-template-reference-overview.html).
 
-#### Intermediate Types
+### Intermediate Types
 
 Intermediate Types are defined by Graphql Types and Fields. They have a set of defined
 fields, where each field corresponds to another type in the system. Intermediate
@@ -703,7 +703,7 @@ const node = new appsync.InterfaceType('Node', {
 
 To learn more about **Interface Types**, read the docs [here](https://graphql.org/learn/schema/#interfaces).
 
-##### Object Types
+#### Object Types
 
 **Object Types** are types that you declare. For example, in the [code-first example](#code-first-example)
 the `demo` variable is an **Object Type**. **Object Types** are defined by
@@ -775,7 +775,7 @@ You can create Object Types in three ways:
 
 To learn more about **Object Types**, read the docs [here](https://graphql.org/learn/schema/#object-types-and-fields).
 
-### Enum Types
+#### Enum Types
 
 **Enum Types** are a special type of Intermediate Type. They restrict a particular
 set of allowed values for other Intermediate Types.
@@ -832,7 +832,7 @@ api.addType(review);
 
 To learn more about **Input Types**, read the docs [here](https://graphql.org/learn/schema/#input-types).
 
-### Union Types
+#### Union Types
 
 **Union Types** are a special type of Intermediate Type. They are similar to
 Interface Types, but they cannot specify any common fields between types.
@@ -860,7 +860,7 @@ api.addType(search);
 
 To learn more about **Union Types**, read the docs [here](https://graphql.org/learn/schema/#union-types).
 
-#### Query
+### Query
 
 Every schema requires a top level Query type. By default, the schema will look
 for the `Object Type` named `Query`. The top level `Query` is the **only** exposed
@@ -883,7 +883,7 @@ api.addQuery('allFilms', new appsync.ResolvableField({
 
 To learn more about top level operations, check out the docs [here](https://docs.aws.amazon.com/appsync/latest/devguide/graphql-overview.html).
 
-#### Mutation
+### Mutation
 
 Every schema **can** have a top level Mutation type. By default, the schema will look
 for the `ObjectType` named `Mutation`. The top level `Mutation` Type is the only exposed
@@ -906,7 +906,7 @@ api.addMutation('addFilm', new appsync.ResolvableField({
 
 To learn more about top level operations, check out the docs [here](https://docs.aws.amazon.com/appsync/latest/devguide/graphql-overview.html).
 
-#### Subscription
+### Subscription
 
 Every schema **can** have a top level Subscription type. The top level `Subscription` Type
 is the only exposed type that users can access to invoke a response to a mutation. `Subscriptions`