Add docs source selector for manual publishes

[jeffhandley]

Summary

• Add a manual Docs workflow source selector.
• Default manual rebuilds to published release tags for post-release deployment retries.
• Allow an all-major refresh from current source branches without per-major overrides.

Validation

• Validated workflow YAML, Bash syntax, and both source-selection modes.
• Full .NET build/test could not complete because api.nuget.org TLS handshakes fail in this environment (NU1301).

Note

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[jeffhandley]

Note that this does not store any sort of marker for where docs have been released from. So a docs publishing followed by another release will have the undesired effect. I want to accept that behavior for the time-being while I think about what behavior I really want to have for the scenario.

This unblocks getting the updated Roadmap docs published.