lots of small doc fixes

[minrk]

• fix lots of broken references (closes 24.0.0: sphinx warnings reference target not found #1767)
• only modify sys.path for buildutils import (closes 22.2.1: issue with building documentation #1621)
• rst2myst for remaining rst files
• add from __future__ import annotations, which enables some fixed annotation resolution
• various docstring fixes to avoid unresolved targets
• remove some very old notes docs about Python 2.5 / 3.0. They still exist in the archives, but don't need to be in the latest docs
• test docs on CI with sphinx-build -W to try to keep warnings down