Docs: Add sphinxcontrib-mermaid for diagram migration

Docs: Add sphinxcontrib-mermaid for diagram migration

Add sphinxcontrib-mermaid alongside existing blockdiag/seqdiag
extensions to prepare for migrating live diagrams from blockdiag
to Mermaid syntax.

The blockdiag/seqdiag extensions are abandoned and incompatible
with modern Python. The live diagram content in this repository
will be migrated to Mermaid syntax in a follow-up change, after
which blockdiag/seqdiag can be removed.

Issue-ID: CIMAN-33
Change-Id: Ibc24e29d11d611c1e29a3e783c483a2667edf42a
Signed-off-by: Claude <noreply@anthropic.com>
Signed-off-by: Matthew Watkins <mwatkins@linuxfoundation.org>