fix: cancel duplicate sphinx builds on consecutive pushes to the same branch

[Copilot]

Consecutive pushes to master or stable* branches would each spawn independent workflow runs with no cancellation, since github.head_ref is empty on push events and the fallback github.run_id is unique per run — making the concurrency group useless for push.

Change

# Before — push events always get a unique group, never cancelled
group: build-documentation-${{ github.head_ref || github.run_id }}

# After — push events group by ref (e.g. refs/heads/master), cancels stale runs
group: build-documentation-${{ github.head_ref || github.ref }}

• PRs: behaviour unchanged — github.head_ref is the PR branch name
• Push: falls back to github.ref, grouping runs by branch and cancelling the in-progress one on the next push