Chore: Modernise docs build configuration

Chore: Modernise docs build configuration

Remove broken sphinxcontrib-swaggerdoc module, drop conflicting
OpenStack Yoga constraints, update to Python 3.11, and align
requirements-docs.txt with the doc repository standards.

The OpenStack Yoga upper-constraints pinned Sphinx===4.4.0 which
conflicts with the ONAP upper-constraints pinning sphinx===7.1.2,
causing pip resolution failures in CI.

Also work around blockdiag incompatibility with Pillow>=10 by
force-downgrading Pillow in commands_pre. blockdiag uses the
ImageDraw.textsize() method that was removed in Pillow 10.0.
Python 3.11 is used (not 3.12) as Pillow<10 has no pre-built
wheels for Python 3.12.

Issue-ID: CIMAN-33
Change-Id: Ie5cb0a4b5c0fd2f06394443a57b0d242c589175e
Signed-off-by: Matthew Watkins <mwatkins@linuxfoundation.org>