Overhaul Pydantic docs

[dantownsend]

Piccolo API has great Pydantic support, but the docs are a bit hidden away under the 'CRUD' section.

It would be good to move them, so 'Serialisation' is a top level topic in the docs, and contains much more examples about how to use create_pydantic_model.

[sinisaos]

@dantownsend Since Pydantic serialization is now in the main Piccolo ORM repo. I can move 'Serialisation' docs (and add examples) from Piccolo API to Piccolo ORM and make a link to those docs in the Piccolo API serializers. What do you think?

[dantownsend]

@sinisaos Yes, I totally agree.

[sinisaos]

@dantownsend I'll make PR for Piccolo and Piccolo API to change docs.

[sinisaos]

@dantownsend Can you please close this issue because it is done in main Piccolo ORM repo.

[dantownsend]

@sinisaos Yep - good point