deploy docs on manual trigger (#462)

It turns out that we are building docs but not deploying docs when we merge
napari/napari PRs, and also on manually-triggered workflows. Only merges to
main on the napari/docs repo get deployed.

This PR also adds workflow_dispatch to the conditions in which built docs are
deployed. (This is also the mechanism by which merges in napari/napari trigger
docs building in the this repo.)

.github/workflows/build_and_deploy.yml

@@ -114,7 +114,7 @@ jobs:
           fi
 
       - name: Deploy Docs
-        if: github.event_name == 'push' && (startsWith(github.ref, 'refs/tags/v') || startsWith(github.ref, 'refs/heads/main'))
+        if: (github.event_name == 'push' || github.event_name == 'workflow_dispatch') && (startsWith(github.ref, 'refs/tags/v') || startsWith(github.ref, 'refs/heads/main'))
         uses: peaceiris/actions-gh-pages@v3
         with:
           deploy_key: ${{ secrets.ACTIONS_DEPLOY_KEY }}