ci: create GitHub releases from version tags (leanprover-community#35425)

## Summary

This PR adds a GitHub Actions workflow that automatically creates GitHub releases when version tags are pushed.

Currently mathlib4 only has tags (no GitHub releases), which makes it harder to programmatically discover the latest release version. This mirrors the existing functionality in batteries' `docs-release.yml`, but without building/attaching docs.

See also leanprover/cslib#340

Closes leanprover-community#11292

🤖 Prepared with Claude Code

Author: kim-em

.github/workflows/release.yml

@@ -0,0 +1,21 @@
+name: Release
+
+on:
+  push:
+    tags:
+      - "v[0-9]+.[0-9]+.[0-9]+"
+      - "v[0-9]+.[0-9]+.[0-9]+-rc[0-9]+"
+
+permissions:
+  contents: write
+
+jobs:
+  release:
+    runs-on: ubuntu-latest
+    if: github.repository == 'leanprover-community/mathlib4'
+    steps:
+      - name: Create GitHub Release
+        uses: softprops/action-gh-release@a06a81a03ee405af7f2048a818ed3f03bbf83c7b # v2.5.0
+        with:
+          prerelease: ${{ contains(github.ref, 'rc') }}
+          make_latest: ${{ !contains(github.ref, 'rc') }}