Allow the documentation website to be indexed by search engines (#5623)

Our HTML documentation template slaps a `<META NAME="ROBOTS"
CONTENT="NOINDEX, NOFOLLOW">` on every page unless the `HUGO_ENV`
environment variable is set to `production`, thereby excluding the
entire documentation website from web search results. Set the variable
to fix it.

This behavior is inherited from Docsy, although they have changed it
since: <https://github.com/google/docsy/pull/653>.

.github/workflows/main.yml

@@ -399,6 +399,8 @@ jobs:
           pip install -r site/requirements.txt
           python site/process_sdk_docs.py --input-dir /tmp/cvat_sdk/docs/ --site-root site/
           python site/build_docs.py
+        env:
+          HUGO_ENV: production
 
       - name: Deploy
         uses: peaceiris/actions-gh-pages@v3