- Added `tax_exempt_reason` and `default_auto_renewal_profile_id` to `Customer`
- Added `received_on` to `InvoicePayment` and `CreateInvoicePayment`
- Added `product_price_point_id` and `product_price_point_handle` to `UpdateSubscription`
- Improved some properties descriptions

* Added scheduleSubscriptionCancellationEnabled property to the Site model
* Added string representations for models

This pull request updates the Maxio Advanced Billing SDK, focusing on
improving documentation, updating the SDK version, and enhancing API
response handling. Key changes include upgrading the SDK version to
`v0.7.0`, adding a new `ApiResponse` utility, and refining documentation
for better clarity and consistency.

### SDK Version Update:
* Updated the SDK version to `v0.7.0` in `README.md`, `client.go`, and
associated documentation. This includes updating the `go get` command
and user-agent string.
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### Documentation Enhancements:
* Improved API client initialization examples by adding `package main`
and `import` statements for better usability. Changes were made in
`README.md`, `doc/auth/basic-authentication.md`, and `doc/client.md`.
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* Standardized parameter formatting in tables across documentation
files, such as `README.md` and `doc/client.md`, by removing backticks
from parameter names.
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* Added a new section for `ApiResponse` in `doc/api-response.md`,
explaining its structure, properties, and usage.

### Code and Comments Improvements:
* Updated error handling in `coupons_controller.go` to include a
templated error message for 404 responses.
* Fixed a typo in `custom_fields_controller.go` by correcting
"non-existant" to "non-existent."

### Codebase Maintenance:
* Replaced multi-line comments with single-line comments for package
headers in all Go files, ensuring consistency and readability.
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### 📝 Changelog: API Documentation Updates

We've made several updates to the API documentation to improve clarity,
consistency, and accuracy. This release includes enhancements to the
descriptions for various endpoints, corrections to example data, and
updates to external links.

---

### 🌐 Webhooks API Improvements

* **Simplified descriptions:** We've streamlined the descriptions for
several webhook endpoints, including **List Webhooks**, **Enable
Webhooks**, **Replay Webhooks**, **Create Endpoint**, **Update
Endpoint**, and **List Endpoints**, to make them more concise and easier
to understand.
* **Updated documentation links:** All internal and external links
within the webhook documentation have been updated to point to the
correct, current resources.

---

### 💰 Billing and Subscription Updates

* **Enhanced Customer API:** The **Read Customer** endpoint now includes
a complete example response, providing a clearer picture of the data
you'll receive from the API.
* **Component Price Point examples:** We've added new examples for
**Volume** and **Tiered** price points to the **Create Component Price
Point** endpoint, providing clearer guidance for different pricing
schemes. The **Stairstep Price Point** example was also corrected to
include a valid `ending_quantity`.
* **Flexible Subscription Usage:** The `subscription_id` path parameter
for the **Create Usage** endpoint has been updated to
`subscription_id_or_reference`. This allows you to use either the
subscription's ID or its reference string, making the endpoint more
flexible.
* **Updated documentation links:** The links in the documentation for
**GoCardless**, **Stripe SEPA/BECS Direct Debit**, and **Chargify.js**
have been corrected to ensure they point to the latest and most relevant
documentation.

---

### 🧰 General Enhancements

* **Concise descriptions:** We've shortened descriptions for the **Read
Customer** and **Read Component** endpoints to be more direct.
* **Improved link consistency:** All external links, especially those
referencing **Chargify.js** and various examples, have been reviewed and
updated to the correct, canonical URLs.