Allow customizing deploy subfolder

[ericphanson]

This way a repo with multiple packages can have independent documenter docs for each package, each deploying to a different subfolder on gh-pages. While more complicated than having a single set of documenter docs for all the packages in the repo, this setup would allow using tags like mypackage/v0.1.0 to tag each package separately, and thus have stable docs for each package (pointing to different tags on the repo).

[fredrikekre]

This is already possible with the dirname keyword argument to deploydocs (Julias manual deploys to /en, for example).

[ericphanson]

Oh that's great, I didn't realize. Thanks!

[fredrikekre]

The tag part doesn't work though (xref JuliaRegistries/TagBot#157).

[ericphanson]

Yeah, my thought is we could at least try doing tags manually until that's fixed