[CD] Request permissions to push to gh-pages branch.

The GITHUB_TOKEN used to have lots of privileges, but this has now
been locked down, so we need to explicitly request write permission
for contents to be able to push to the gh-pages branch.

.github/workflows/main.yml

@@ -14,6 +14,9 @@ jobs:
   build-deploy:
     name: Build CIRCT website
     runs-on: ubuntu-latest
+    # Request permissions to push to gh-pages branch.
+    permissions:
+      contents: write
     steps:
       - uses: actions/checkout@v1