gh-153906: Modernize pydoc HTML output (GH-153909)

gh-153906: Modernize pydoc HTML output (GH-153909)

Follow-up of the HTML5 migration in bpo-10716: replace the layout
tables, the &nbsp;/<br> encoding of docstrings and the obsolete
<a name=...> anchors with semantic markup (header, section, h1-h4,
CSS multi-column lists, white-space: pre-wrap, id= anchors), and
restyle the pages after the python-docs-theme used by docs.python.org,
including dark mode support.  Members inherited from other classes
are now collapsed by default.  The pydoc API is unchanged; obsolete
arguments of HTMLDoc methods are accepted and ignored.  ServerHTMLDoc
in xmlrpc.server is updated to match.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>