asf-site branch should just force push

[utzig]

Currently a PR is generated on asf-site branch every time the site is rebuilt and merged manually after review. Since everything is auto-generated and the output size is somewhat big, this is a very expensive operation to keep doing. asf-site should only ever have the latest version, so the latest build should always force push itself and be the only commit in this branch. If the site breaks for some reason, the fixes should go to the docs themselves or to the site master (theme, docs, whatever).