D8 fleet sync: docfx.yaml from D20 canonical + delete picker-template.html

[Chris-Wolfgang]

Summary

Brings this repo to the D8 fleet canonical state matching D20-Dice:

1. docfx.yaml — sync from Chris-Wolfgang/D20-Dice@main:.github/workflows/docfx.yaml. The root index.html generation step is rewritten to emit an inline meta-refresh → /versions/latest/ instead of templating from .github/version-picker-template.html. The workflow is repo-agnostic (reads repo name from $GITHUB_REPOSITORY at runtime).
2. .github/version-picker-template.html — delete. No longer referenced by docfx.yaml after this PR; the in-page picker (canonical docfx_project/public/version-picker.js) handles version switching from every docs page.

build-all-versions.yaml retains its reference to the HTML file — this matches D20 / DateTime-Extensions / ICollection-Extensions etc. — fleet-wide dead reference in the manual-rebuild path, not in scope here.

Rationale (from D20's canonical comment)

The in-page version picker handles version switching from any docs page now, so the root no longer needs to display a picker UI — a meta-refresh + JS fallback + <noscript> link is all that's needed. Inline beats template-file because the only dynamic piece is the page title.

Test plan

Detect .NET Projects will fire on this PR — that's the protected-files guard, expected. Admin-bypass merge after maintainer review.

• D20 docfx.yaml has no D20-specific strings (verified with grep -niE "D20.Dice|Wolfgang\.D20" — empty)
• CI: Detect .NET Projects ❌ expected; gitleaks / DevSkim / etc. ✅
• Admin-bypass merge after manual review
• Next gh-pages deploy: root / becomes a meta-refresh to /versions/latest/

Part of the D8 fleet cleanup. Companion PRs in IAsyncEnumerable-Extensions: #257 + #256.

🤖 Generated with Claude Code

[Chris-Wolfgang]

Superseded by #195 (canonical/d8-inline-root-redirect). Both PRs do the same canonical D8 work — delete version-picker-template.html and refactor docfx.yaml to inline the redirect HTML. #195 is more complete: also touches build-all-versions.yaml and docs/DOCFX-VERSION-PICKER.md, and carries the more precise step-name wording. Closing this duplicate.