Update the branches page with more information on branching practices

Add more information on branch naming and protected branches.

Mention "master" equally to "main" since both names are used roughly equally, and nobody is currently working on renaming, and it's unlikely that GNOME will settle on one name or the other anytime soon. See Infrastructure/Infrastructure#788 for more information regarding this.

Provide more options for when to create stable branches. This fixes #44 (closed).