🔧 site.yml: Change default branch to `main`

This commit changes the default branch to main instead of master. This makes for one less step that someone has to remember if they are using a new git repository. It has been a long time since master was the default.

Additionally, I copied the file from production, so several other docs sites are also now built in the staging site too (I think?). I have not verified the new additions very closely however.