Docs: Replace blockdiag/seqdiag with Mermaid

Docs: Replace blockdiag/seqdiag with Mermaid

Remove sphinxcontrib-blockdiag and sphinxcontrib-seqdiag from all
configuration files, requirements, and constraints. Add
sphinxcontrib-mermaid as the replacement.

Changes across docs/conf.py, example templates, and constraints:
- Replace blockdiag/seqdiag extensions with sphinxcontrib.mermaid
- Remove Pillow dependency (only needed for blockdiag rendering)
- Drop setuptools<82 cap (only needed for blockdiag pkg_resources)
- Remove blockdiag family from OS constraints (blockdiag, seqdiag,
  actdiag, nwdiag, funcparserlib, and their sphinxcontrib wrappers)
- Bump tox.ini basepython to 3.13 (matches .readthedocs.yaml)

Verified: pip install succeeds on Python 3.13 with zero
blockdiag/seqdiag/Pillow packages. sphinx-build completes
with no errors or warnings using -W flag.

Issue-ID: CIMAN-33
Co-Authored-By: Claude <noreply@anthropic.com>
Change-Id: If9f3aafb387972d057d98f60cfb6d79b5136000c
Signed-off-by: Matthew Watkins <mwatkins@linuxfoundation.org>