Set default branch to gh-pages

[nickserv]

GitHub Pages will automatically be enabled if you push a gh-pages branch to a repository, so I think this action should be able to just do that by default to make configuration simpler. The similar peaceiris/actions-gh-pages action already does this.

[nickserv]

Would a PR be helpful or do we want to reach a consensus first?

[JamesIves]

I think this would be a useful PR if you'd like to make it!

[nickserv]

This is done.