Fix RTD build and remove unmaintained extensions

Fix RTD build and remove unmaintained extensions

Run download-specs.sh in RTD pre_build job so the
OpenAPI spec file is present for sphinxcontrib-openapi.
Remove abandoned blockdiag/seqdiag/swaggerdoc extensions
and their dependencies (Pillow, six). Mermaid was already
added in the preceding change. Update build to
ubuntu-24.04 and Python 3.13.

Issue-ID: CIMAN-33
Co-Authored-By: Claude <noreply@anthropic.com>
Change-Id: I63aec35d9cd4530bda7213317218ea1af5cbce29
Signed-off-by: Matthew Watkins <mwatkins@linuxfoundation.org>