Docs: Add migration guide from blockdiag/seqdiag to Mermaid

Docs: Add migration guide from blockdiag/seqdiag to Mermaid

sphinxcontrib-blockdiag and sphinxcontrib-seqdiag have not been maintained
since December 2021 and are incompatible with Python 3.12+
(ast.NameConstant removal) and Pillow 10+ (ImageDraw.textsize removal).

This adds a comprehensive documentation guide that:
- Details the problems with the current blockdiag/seqdiag extensions
- Evaluates alternatives (Mermaid, PlantUML, Graphviz)
- Recommends sphinxcontrib-mermaid as the replacement
- Provides step-by-step migration instructions for repositories
- Includes syntax conversion examples for both block and sequence
  diagrams
- Lists the changes needed in doc repo files (conf.py,
  requirements-docs.txt, upper-constraints.onap.txt)
- Provides a downstream repository migration checklist

Issue-ID: CIMAN-33
Change-Id: I3b6ce0eb31fca0f9ea36cbca75575b3ba6a33ea4
Signed-off-by: Matthew Watkins <mwatkins@linuxfoundation.org>