Docs: Modernise docs build and replace swaggerdoc with openapi

Docs: Modernise docs build and replace swaggerdoc with openapi

Replace deprecated sphinxcontrib-swaggerdoc with sphinxcontrib-openapi.
Replace swaggerv2doc directive in sim-participant.rst with openapi.

Modernise docs build configuration:
- Remove OpenStack Yoga constraints (already removed in prior change)
- Update basepython from 3.8 to 3.12
- Add setuptools>=65.0.0 as explicit dependency
- Update .readthedocs.yaml to ubuntu-24.04 and python 3.12
- Align requirements-docs.txt versions with doc repo standards
- Add allowlist_externals to docs testenv

Issue-ID: CIMAN-33
Change-Id: I1332eb23d386354d21c0049471c3ad781a929224
Signed-off-by: Matthew Watkins <mwatkins@linuxfoundation.org>