✨ Add reference (code API) docs with PEP 727, add subclass with custom docstrings...

✨ Add reference (code API) docs with PEP 727, add subclass with custom docstrings for `BackgroundTasks`, refactor docs structure (#10392)

* ➕ Add mkdocstrings and griffe-typingdoc to dependencies

* 🔧 Add mkdocstrings configs to MkDocs

* 📝 Add first WIP reference page

* ⬆️ Upgrade typing-extensions to the minimum version including Doc()

* 📝 Add docs to FastAPI parameters

* 📝 Add docstrings for OpenAPI docs utils

* 📝 Add docstrings for security utils

* 📝 Add docstrings for UploadFile

* 📝 Update docstrings in FastAPI class

* 📝 Add docstrings for path operation methods

* 📝 Add docstring for jsonable_encoder

* 📝 Add docstrings for exceptions

* 📝 Add docstsrings for parameter functions

* 📝 Add docstrings for responses

* 📝 Add docstrings for APIRouter

* ♻️ Sub-class BackgroundTasks to document it with docstrings

* 📝 Update usage of background tasks in dependencies

* ✅ Update tests with new deprecation warnings

* 📝 Add new reference docs

* 🔧 Update MkDocs with new reference docs

* ✅ Update pytest fixture, deprecation is raised only once

* 🎨 Update format for types in exceptions.py

* ♻️ Update annotations in BackgroundTask, `Annotated` can't take ParamSpec's P.args or P.kwargs

* ✏️ Fix typos caught by @pawamoy

* 🔧 Update and fix MkDocstrings configs from @pawamoy tips

* 📝 Update reference docs

* ✏️ Fix typos found by @pawamoy

* ➕ Add HTTPX as a dependency for docs, for the TestClient

* 🔧 Update MkDocs config, rename websockets reference

* 🔇 Add type-ignores for Doc as the stubs haven't been released for mypy

* 🔥 Remove duplicated deprecated notice

* 🔇 Remove typing error for unreleased stub in openapi/docs.py

* ✅ Add tests for UploadFile for coverage

* ⬆️ Upgrade griffe-typingdoc==0.2.2

* 📝 Refactor docs structure

* 🔨 Update README generation with new index frontmatter and style

* 🔨 Update generation of languages, remove from top menu, keep in lang menu

* 📝 Add OpenAPI Pydantic models

* 🔨 Update docs script to not translate Reference and Release Notes

* 🔧 Add reference for OpenAPI models

* 🔧 Update MkDocs config for mkdocstrings insiders

* 👷 Install mkdocstring insiders in CI for docs

* 🐛 Fix MkDocstrings insiders install URL

* ➕ Move dependencies shared by docs and tests to its own requirements file

* 👷 Update cache keys for test and docs dependencies

* 📝 Remove no longer needed __init__ placeholder docstrings

* 📝 Move docstring for APIRouter to the class level (not __init__ level)

* 🔥 Remove no longer needed dummy placeholder __init__ docstring