Allow deploy_to_branch() to deploy to a configurable subdirectory of the branch

[gadenbuie]

This is a small PR to add a subdir argument to deploy_to_branch(). By default (as currently) deploy_to_branch() deploys the packge documentation into the root of a branch, typically an orphan gh-pages branch.

When subdir is provided, deploy_to_branch() will set up the git worktree the same as before, but will build the site into the requested subdir of the branch. This enables a variety of site structures, but the two I'm most excited about are:

1. Enabling PR previews via GitHub Actions by building the site preview into preview/pr${{ github.event.pull_request.number }}.

2. Enabling documentation for multiple package versions, for example based on tags, by building the site into separate folders, e.g. subdir = "v2" and subdir = "v3".

As part of the process, we also append the subdir to pkg$meta$url, e.g. https://pkg.example.com/v2. To facilitate that change, I moved the as_pkgdown() call into deploy_to_branch(), which required also overriding the destination here rather than in build_site_github_pages().

[hadley]

Do you want to add a news bullet? I'm not convinced this is the direction we want to go in the long term (since I think it might be better for us to use same GitHub pages deployment for bookdown, blogdown, ...) but I don't think it's harmful to include it and it certainly seems like it will enable some interesting experiments.

[gadenbuie]

Just added a news bullet! I'm looking forward to a consolidated GitHub Pages deployment, but I'll also be glad to have this feature. By the way, the use of git worktree is quite clever and I learned a lot from looking into this.

[hadley]

Thanks!