docs: sphinx integration for mintlify docs

[rmad17]

Adds Sphinx-based API reference documentation infrastructure for the auth0-fastapi SDK. The generated JSON artifact is consumed by the auth0.com/docs Mintlify site to render the SDK reference at /docs/sdk/python/fastapi.

Changes made

Sphinx docs setup (docs/)

• conf.py — Sphinx configuration using sphinx-autoapi (AST-based, no import required), furo theme with Auth0 brand colors, Napoleon for Google-style docstrings, intersphinx mapping to Python and Pydantic docs. Includes a autoapi-skip-member hook to suppress Pydantic internals (BaseModel.Config, auto-generated __init__ docstrings) from the output.
• index.rst — Root document pointing to the autoapi-generated reference.
• Makefile — Build targets: html (local preview), json (Mintlify artifact), clean.
• requirements.txt — Pinned build dependencies: sphinx, sphinx-autoapi, furo.
• _static/auth0.css — Auth0 brand colors and Pydantic-style parameter block styling for local HTML preview.
• _templates/autoapi/python/module.rst — Custom page title template that uses the module's short name (e.g. "Auth Client") instead of the full dotted path.

Source changes

• src/auth0_fastapi/__init__.py — Added empty init to ensure autoapi resolves modules as auth0_fastapi.* rather than src.auth0_fastapi.*.
• src/auth0_fastapi/auth/auth_client.py — Improved docstrings for DX:
  • RFC 8693 references converted to RST hyperlinks pointing to rfc-editor.org.
  • Return types (TokenExchangeResponse, LoginWithCustomTokenExchangeResult, SessionTransferTokenResult) marked up with :class: for cross-linking.
  • Exception names in Raises sections (CustomTokenExchangeError, InvalidArgumentError, MissingRequiredArgumentError, ValueError) marked up with :exc:.
  • Inline parameter names wrapped in double backticks for monospace rendering.

.gitignore

• Replaced the broad docs ignore with targeted entries (docs/_build/, docs/reference/) so the Sphinx source files are tracked while build output is not.

Generating the artifact

cd docs
pip install -r requirements.txt
make json
# Output: _build/json/ — copy to docs-v2/main/sdk-artifacts/auth0-fastapi/

Testing

- Run make json and verify _build/json/ is produced with no errors.
- Run make html and open _build/html/index.html for a local preview (Auth0 colors, clean sidebar, RFC hyperlinks, cross-referenced types).