Write documentation on using the smithy4s json module directly

[a-morales]

PR Checklist (not all items are relevant to all PRs)

• Added unit-tests (for runtime code)
• Added bootstrapped code + smoke tests (when the rendering logic is modified)
• Added build-plugins integration tests (when reflection loading is required at codegen-time)
• Added alloy compliance tests (when simpleRestJson protocol behaviour is expanded/updated)
• Updated dynamic module to match generated-code behaviour
• Added documentation
• Updated changelog

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[lewisjkl]

Just a few minor clarifications here on the wording and then also I think we should add the following warning to these docs as well

smithy4s/modules/json/src/smithy4s/json/Json.scala

Line 36 in 7cd62db

* Beware : using this method with a non-static schema (for instance, dynamically generated) may

[Baccata]

Gentle reminder that we already have documentation regarding serialisation in different formats : https://disneystreaming.github.io/smithy4s/docs/02.1-serialisation/serialisation#json

It may be valuable to split this page into different pages (per serialisation format), and possibly to re-order the entries in the sidebar such that the serialisation section appears higher. The contents of this PR, if needed, should go in the JSON specific page.

[plokhotnyuk]

If you would like to enhance description for customization then please see here a lot of examples of smithy4s-json custom codecs for efficient working with arrays and tuples, skipping default values of case class fields, etc.