Docs: Update migration report, align Python 3.13

Docs: Update migration report, align Python 3.13

- Rewrite blockdiag/seqdiag migration report as a completed technical
  reference rather than a TSC progress tracker
- Remove Audit results, Gerrit changes, Ask of the TSC, and Timeline
  sections containing moment-in-time tracking content
- Remove Gerrit change references throughout
- Add pre-rendered blockdiag/seqdiag PNG images for Before diagrams
  (the extensions are no longer installed so code-block source alone
  was not rendering visually)
- Fix broken Bus Controller mermaid diagram: quote hyphenated node
  identifiers and rename classDef names to avoid reserved keywords
- Add new Migration scope, Mermaid syntax pitfalls, and Applying the
  migration to other projects sections
- Update example tox.ini templates (master and release) from
  Python 3.8 to 3.13, remove OpenStack Yoga constraints, add
  setuptools

Issue-ID: CIMAN-33
Change-Id: Ie0612c80cb1632771f537665b1f013daf77ac80d
Signed-off-by: Matthew Watkins <mwatkins@linuxfoundation.org>