docs: Rework the documentation

[Kriskras99]

TODO:

• Proofread everything
• Split the Clippy lints and fixing them into a separate commit

The first page of our documentation was overwhelmingly large. This commit moves the documentation to several places:

• The introduction to Avro itself was moved to documentation::primer
• Using the crate the "Avro" way was moved to documentation::generic
  • I've also changed the focus to the fact that it allows for more dynamic code
• Using the crate the "Serde" way was moved to serde
• Calculating schema fingerprints was moved to Schema::fingerprint
  • Users can easily find it by searching for fingerprint
• Custom name validators was moved to validator
  • Can be found via search and listed under the modules on the first page
• Custom schema equality was moved to schema_equality
  • Can be found via search and listed under the modules on the first page It also removes some sections
• How to install the library and enable features, this is basic Rust knowledge
• Section about breaking changes in minor versions, this is to be expected as the crate is at 0.*
• Reading and writing logical types, this was a very large section but not very useful
• Ill formed data, instead the error message was improved by adding a link to the function to change the limit

This also adds documentation for the AvroSchema derive (on the trait) and updates the derive crate documentation to link to it.

In addition, I've enabled some Clippy lints related to documentation and fixed all the lints it caused.

Future work:

• Enable the clippy.missing_errors_doc and clippy.missing_panics_doc lints
• Deprecate some types that live in the root namespace
  • For example, the codec related types. Instead the codec module should be public
  • This will further improve the signal to noise ratio on the first page