Docs: Replace blockdiag/seqdiag with Mermaid

Docs: Replace blockdiag/seqdiag with Mermaid

Replace the abandoned sphinxcontrib-blockdiag and sphinxcontrib-seqdiag
Sphinx extensions with sphinxcontrib-mermaid. Remove Pillow dependency
which was only required for blockdiag server-side image rendering.

Mermaid renders diagrams client-side via JavaScript, requiring no
system packages or native image libraries.

Bump basepython from python3.12 to python3.13 to match
.readthedocs.yaml — blockdiag was the blocker preventing this
(uses removed ast.NameConstant on 3.12+, and pkg_resources on
setuptools 82+).

None of the documentation in this repo uses blockdiag or seqdiag
directives; this is a config-only cleanup.

Issue-ID: CIMAN-33
Change-Id: Idb48cffb4b6cfcb8ac76359daa3f59a4d0435bb9
Signed-off-by: Claude <noreply@anthropic.com>
Signed-off-by: Matthew Watkins <mwatkins@linuxfoundation.org>