grant write permission to publish_docs

Author: Musab1258

.github/workflows/nightly_docs.yml

@@ -34,6 +34,8 @@ jobs:
     if: github.ref == 'refs/heads/master'
     needs: [build_docs]
     runs-on: ubuntu-latest
+    permissions:
+      contents: write
     steps:
       - name: Checkout `bitcoindevkit.org`
         uses: actions/checkout@v4