Docs: Modernise docs build configuration

Docs: Modernise docs build configuration

Follow-up to the merged swaggerdoc removal (143028). Modernise the
remaining docs build configuration:
- 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
- Uncomment docs-linkcheck testenv (Yoga constraints removed)
- Add allowlist_externals to docs testenv
- Comment out swaggerv2doc directive in bp-processor.rst (source
  JSON file contains malformed JSON that no Sphinx directive can parse)
- Remove orphaned test_test_test label in node-type.rst that causes
  duplicate target error with docs-linkcheck

Issue-ID: CIMAN-33
Change-Id: I87199552ab5589ada4dec3a6f80f3f941bb32f01
Signed-off-by: Matthew Watkins <mwatkins@linuxfoundation.org>