Ignore superpowers-generated docs directory (#48633)

## Summary

Add `docs/superpowers/` to `.gitignore` so all locally generated
superpowers artifacts (specs, design docs, and plans) are not committed.

These files are produced by local AI tooling and are workspace-local
only; they should not land in the repository.

## Change

```gitignore
# Superpowers-generated docs (specs, design, plans) — local-only, not committed
docs/superpowers/
```

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Yu Leng <yuleng@microsoft.com>
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.gitignore

@@ -378,3 +378,6 @@ installer/*/*.wxs.bk
 vcpkg_installed/
 
 deps/vcpkg/
+
+# Superpowers-generated docs (specs, design, plans) — local-only, not committed
+docs/superpowers/