schedule-doc-report: add workflow_dispatch

https://docs.github.com/en/actions/using-workflows/manually-running-a-workflow

.github/workflows/schedule-doc-report.yml

@@ -2,6 +2,7 @@ name: Generate documentation versions report
 on:
   schedule:
     - cron: '17 4 1 * *'
+  workflow_dispatch:
 
 jobs:
   report: