ci: unify release flow into single workflow

[abosnjakovic]

Summary

• Collapse release-please.yml, release.yml, and docs.yml into a single release.yml triggered on push to main
• Chain steps via release-please-action outputs: bump version → tag → regenerate vimdoc → force-move tag onto vimdoc commit → create GH release with changelog excerpt → LuaRocks upload
• vimdoc is now regenerated only at release time (no more per-push chore(ci): auto-generate vimdoc churn on main)

Why

Release was split across three workflows linked by triggers (push-to-main → tag-push). User-facing it felt like several actions. Now a single workflow run from merging the release-please PR carries the release end-to-end.

Notes

• Tag force-move is safe: nothing consumes vX.Y.Z in the seconds between release-please creating it and the vimdoc follow-up commit
• GITHUB_TOKEN-authored pushes do not retrigger workflows, so the vimdoc commit will not loop
• The pending chore(main): release 1.0.1 PR was opened under the old workflow set; the unified flow takes effect from the next release cycle

Test plan

• Merge a throwaway feat: commit, confirm only ci + release workflows appear
• Merge the resulting release-please PR
• Confirm the single release run: tags vX.Y.Z, commits regenerated vimdoc, force-moves the tag, creates GH release with extracted changelog, publishes to LuaRocks
• Verify doc/nyan.txt at the tag matches generated output