add api docs for pydantic-core

[samuelcolvin]

Needs:

• a new release of pydantic-core
• i think this needs a fix for mkdocstrings python to avoid all dunder attributes being documented - see docs for compiled modules contain all attributes mkdocstrings/griffe#156

[tpdorsey]

@samuelcolvin you can set the handler options to ignore with filters https://mkdocstrings.github.io/python/usage/#globallocal-options

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[samuelcolvin]

Merging this even though there's more to do, mostly:

• adding docstrings to pydantic_core/_pydantic_core.pyi
• adding each member of pydantic_core/__init__.py individually
• waiting for Error parsing the following code in a .pyi file, where types are marked as strings mkdocstrings/griffe#159
• waiting for docs for compiled modules contain all attributes mkdocstrings/griffe#156 (comment)