Improve Mermaid setup

[nojaf]

List advice guidance to support plain ```mermaid fenced blocks
promoted by a docs/_body.html script, the pattern used by the fantomas
docs. The source now renders on GitHub and the recipe page actually
renders its own diagrams.

[nojaf]

/repo-assist review

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Reviewed this PR as requested. The approach is solid:

• docs/_body.html correctly targets code[lang="mermaid"] (matching how Transformations.fs emits <code lang=""> for fenced code blocks), replaces the closest table.pre/pre wrapper, and uses textContent (not innerHTML) to un-escape the HTML-encoded arrows before mermaid parses them — this is the key detail that makes the pattern work.
• Plain ```mermaid fences now render natively on GitHub and via fsdocs, which is a nice portability win over the old raw <div class="mermaid"> approach.
• CSS addition centers SVG output cleanly; RELEASE_NOTES.md is updated under [Unreleased]; the old docs/sidebyside/sidemermaid.md is removed and its .sln folder entry replaced correctly.
• CI (build ubuntu/windows, Ionide analyzers) is green.

No issues found. One minor, non-blocking thought for follow-up: the CDN import in _body.html has no integrity/fallback if jsdelivr is unreachable, but that mirrors the previous setup and is out of scope here.

Looks good to merge from an automated review perspective — final call left to human maintainers as always.

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