Build the website with github actions

Infra is shutting down Pelican builds with buildbot, ref email to PMCs and wiki https://cwiki.apache.org/confluence/display/INFRA/Moving+from+Infrastructure-pelican+to+GitHub+Actions

I did the switch in https://github.com/apache/solr-site/pull/99 but the site does not build due to missing python modules needed by 'wex' plugin. Reason is that the infra github-action does not currently install custom packages from requirements.txt, like the old buildbot one did. So I filed a PR to add that feature: https://github.com/apache/infrastructure-actions/pull/63