docs: enable nitpicky mode

[brianschubert]

Sphinx has an option to warn about references missing targets, aka nitpicky mode.

Enabling this mode in the docs build reveals the following warnings:

.../typing_extensions/doc/index.rst:305: WARNING: py:class reference target not found: types.NoneType [ref.class]
.../typing_extensions/doc/index.rst:471: WARNING: py:mod reference target not found: typing.TypedDict [ref.mod]
.../typing_extensions/doc/index.rst:527: WARNING: py:class reference target not found: types.NoneType [ref.class]
.../typing_extensions/doc/index.rst:558: WARNING: py:class reference target not found: types.NoneType [ref.class]
.../typing_extensions/doc/index.rst:800: WARNING: py:class reference target not found: types.NoneType [ref.class]
.../typing_extensions/doc/index.rst:1371: WARNING: py:data reference target not found: typing.Union [ref.data]

Spot checks show that these hits are indeed missing links:

• https://typing-extensions.readthedocs.io/en/latest/#:~:text=types.NoneType
• https://typing-extensions.readthedocs.io/en/latest/#:~:text=closer%20to%20the%20implementation%20of-,typing.TypedDict
• https://typing-extensions.readthedocs.io/en/latest/#:~:text=typing.Union

This fixes those cross-references and leaves nitpicky mode enabled in conf.py to help catch similar issues going forward.