PYD-164: Rework custom types docs

[adriangb]

Selected Reviewer: @Kludex

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[linear]

PYD-164 Add docs about GetPydanticSchema in usage/types/custom.md

While doing this, I think it might be good to make a few other tweaks to custom.md (these are just suggestions, not required):

• Rename Composing types via Annotated to Augmenting types with `Annotated`
• Rename Creating custom classes using __get_pydantic_core_schema__ to Using __get_pydantic_core_schema__
  • Make the following subsections of Using __get_pydantic_core_schema__:
    • As a method(or similar)
    • As an annotation(or similar)
    • Handling third party types (move from where it currently is)
    • With `GetPydanticSchema`
• Rename Generic classes as typesto Handling custom generic classes

[adriangb]

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[adriangb]

please review cc @dmontagu

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