Switch from only to rules to determine when to rebuild the website.

Allows the ability to request a website rebuild by using a certain phrase in a commit message.

A commit message containing, build-website should trigger a website rebuild from any branch. The website is always rebuilt for commits to master and next. In all other instances a website rebuild will be a manual action on gitlab.

This provides a convenience, allowing us to update the website from any branch (e.g. for PRs that update the documentation). This also has the downside that anyone with write access can trigger a website rebuild with whatever content is in their branch, which could cause confusion/issues if there are a few people working on website changes etc.