Remove workflow_dispatch for build_doc

As the workflow is already reacting to user actions, there is no need to
trigger it manually. Besides, it requires an `issue_comment` context.

.github/workflows/build_doc.yml

@@ -3,7 +3,6 @@ name: Documentation
 on:
   issue_comment:
       types: [created]
-  workflow_dispatch:
 
 permissions:
   contents: read # to fetch code (actions/checkout)