Docs: Add blockdiag/seqdiag Gerrit repository audit appendix

Docs: Add blockdiag/seqdiag Gerrit repository audit appendix

Audited all ONAP project repositories in Gerrit to determine the blast
radius of the sphinxcontrib-blockdiag and sphinxcontrib-seqdiag
deprecation. Adds an appendix to the existing migration guide with:

- Inventory of repositories containing live blockdiag directives:
  dmaap/buscontroller, dmaap/datarouter, sdc, sdnc/oam (5 directives)
- Inventory of repositories containing live seqdiag directives:
  vnfrqts/requirements (4 directives across 2 VES spec files)
- Summary table: 9 total diagrams requiring migration to Mermaid
- Recommended migration priority order
- Note that 32 additional repositories load the extensions in conf.py
  and requirements-docs.txt but have no actual directives (config-only
  cleanup needed)

Issue-ID: CIMAN-33
Change-Id: I87ddfa32d699a8b4bde99bc4f2cc12f64ca98d36
Signed-off-by: Matthew Watkins <mwatkins@linuxfoundation.org>