ci: restore contents write for mgmt api docs token

The implicit `contents: write` it previously inherited from the App installation.
This led to the weekly mgmt api docs job failing with a 403 once the upstream spec changed.

Add `permission-contents: write` so the token can push the branch again.

PR: https://github.com/supabase/supabase/pull/46954
Closes: https://linear.app/supabase/review/ci-restore-contents-write-for-mgmt-api-docs-token-bb8134ed1ecf
Refs: https://github.com/supabase/supabase/pull/46454
Refs: https://github.com/supabase/supabase/actions/workflows/docs-mgmt-api-update.yml

.github/workflows/docs-mgmt-api-update.yml

@@ -49,6 +49,7 @@ jobs:
           client-id: ${{ vars.GH_AUTOFIX_APP_CLIENT_ID }}
           private-key: ${{ secrets.GH_AUTOFIX_PRIVATE_KEY }}
           permission-pull-requests: write
+          permission-contents: write
 
       - name: Create pull request
         uses: peter-evans/create-pull-request@c5a7806660adbe173f04e3e038b0ccdcd758773c # v6.1.0