GH actions: permissions.contents.write

refs manubot/rootstock#485 (comment)

Some users might have their repository workflow permissions
set to read only. This change should ensure the workflow job
can write commits back to the output and gh-pages branches
on GitHub.

.github/workflows/manubot.yaml

@@ -38,6 +38,8 @@ jobs:
   manubot:
     name: Manubot
     runs-on: ubuntu-latest
+    permissions:
+      contents: write
     env:
       GITHUB_PULL_REQUEST_SHA: ${{ github.event.pull_request.head.sha }}
       # Set SPELLCHECK to true/false for whether to check spelling in this action.