docs/sphinx: add compat.py module and nested_parse helper

docs/sphinx: add compat.py module and nested_parse helper

Create a compat module that handles sphinx cross-version compatibility
issues. For the inaugural function, add a nested_parse_with_titles()
helper that handles differences in line number tracking for nested
directive body parsing.

Spoilers: there are more cross-version hacks to come throughout the
series.

Signed-off-by: John Snow <jsnow@redhat.com>
Message-ID: <20250311034303.75779-5-jsnow@redhat.com>
Acked-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>