ci: allow the docs site deploy to be triggered manually (#2177)

Add a workflow_dispatch trigger to the docs deploy workflow so it can be
re-run on demand. The workflow previously only ran on push to main, so a
dropped push event (as happened with #2171) leaves the change unpublished
with no way to re-trigger the deploy.

Author: andygrove

.github/workflows/docs.yaml

@@ -16,6 +16,9 @@
 # under the License.
 
 on:
+  # Allow the docs site to be rebuilt and republished on demand, e.g. when a
+  # push-triggered run was dropped and the change never made it to asf-site.
+  workflow_dispatch:
   push:
     branches: [ "main" ]
     paths: