Add a brief example

- reST -> MyST
- Roughly group the commands, based on what they work on.

Add a section in the documentation to suggest solutions to the ``pip freeze`` permission denied issue. Update link in source code.

We've created a fairly capable setup with the new resolver tests,
which has now rendered this logic redundant and generally unnecessary.

- Change to Markdown
- Split the Table of Contents

This simplifies the Sphinx configuration and adds additional grouping
and structure to the existing code as well.

See #9612

The typing module has been available since Python 3.5. Guarding the
import has been unnecessary since dropping Python 2.

Some guards remain to either:

- Avoid circular imports
- Importing objects that are also guarded by typing.TYPE_CHECKING
- Avoid mypy_extensions dependency