ci: Add contents permission to publish-docs

Without this permissions the job can't publish GitHub Pages.

Signed-off-by: Leandro Lucarella <leandro.lucarella@frequenz.com>

.github/workflows/ci.yaml

@@ -103,6 +103,8 @@ jobs:
     needs: ["test", "build-dist"]
     if: github.event_name == 'push'
     runs-on: ubuntu-20.04
+    permissions:
+      contents: write
     steps:
       - name: Calculate and check version
         id: mike-metadata