Docs: Fix broken links and modernise extensions

Docs: Fix broken links and modernise extensions

Replace the abandoned sphinxcontrib-blockdiag and sphinxcontrib-seqdiag
Sphinx extensions with sphinxcontrib-mermaid in docs configuration.

Fix broken GitHub URLs in participant-sim.json OpenAPI spec where
the directory was renamed from policy-clamp-participant-impl-simulator
to participant-impl-simulator. Also fix typo in description field.

Fix broken EclipseLink URL in APEX-Policy-Guide.rst.

Issue-ID: CIMAN-33
Change-Id: I488b1ff5a6ef4a33403d305f69b75d5897568c88
Co-Authored-By: Claude <noreply@anthropic.com>
Signed-off-by: Matthew Watkins <mwatkins@linuxfoundation.org>