ci(deploy): allow workflow_dispatch to also trigger Vercel deploy (#34081)

Today's three skills-index PRs (#33748, #33809, #34025) merged to main
but the live Vercel-hosted docs site didn't pick them up — Vercel is
fired by the deploy-vercel job, which was gated on release events only.
Out-of-band main commits between releases couldn't reach Vercel without
cutting a tag.

Widen the gate to also include workflow_dispatch so 'gh workflow run
deploy-site.yml' can ship pending main changes to Vercel on demand.
Release-tag behavior is unchanged.

.github/workflows/deploy-site.yml

@@ -22,7 +22,12 @@ concurrency:
 
 jobs:
   deploy-vercel:
-    if: github.event_name == 'release'
+    # Triggered automatically on release publish (production cuts) and
+    # manually via `gh workflow run deploy-site.yml` when an out-of-band
+    # main commit needs to ship live before the next release tag — e.g.
+    # a skills-index PR that doesn't touch website/** paths and so
+    # doesn't auto-deploy via the deploy-docs path.
+    if: github.event_name == 'release' || github.event_name == 'workflow_dispatch'
     runs-on: ubuntu-latest
     steps:
       - name: Trigger Vercel Deploy