Support any gh-pages branch

[asmeurer]

GitHub now supports any gh-pages branch https://github.blog/changelog/2020-07-31-build-and-deploy-github-pages-from-any-branch-beta/

[asmeurer]

Also it can be root or docs/ in any branch. Not sure if it used to be that way.

[asmeurer]

Maybe this already works, at least for the branch name bit. We have the --deploy-branch-name flag and it doesn't seem to check anywhere that it is 'master' or 'gh-pages'. I thought it did, but all it does is use those as the defaults.

[asmeurer]

So this probably just needs to be documented. And we should double check that it all works.

Also, does the API let you see what the gh-pages branch is? If so, we should use it in doctr configure?

At some point GitHub will probably change the default from master to main for .github.io repos.