Move some MkDocs responsibilities around (#5542)

## Summary

Note that I've also changed from `mkdocs serve` to `mkdocs serve -f
mkdocs.generated.yml` to be clearer that this is a generated file.

.github/workflows/docs.yaml

@@ -26,7 +26,7 @@ jobs:
         run: |
           python scripts/transform_readme.py --target mkdocs
           python scripts/generate_mkdocs.py
-          mkdocs build --strict
+          mkdocs build --strict -f mkdocs.generated.yml
       - name: "Deploy to Cloudflare Pages"
         if: ${{ env.CF_API_TOKEN_EXISTS == 'true' }}
         uses: cloudflare/wrangler-action@2.0.0