Allow to specify a directory to include in html_static_path

[antonio-rojas]

Currently one can specify a directory so that its contents are recursively included in html_static_path, or a specific file to be included. In some cases it would be useful to be able to specify a directory to be included (itself, not just its contents). As far as I can see, there is no way to do that at the moment.

This came up during the recent upgrade to mathjax 3 in sagemath: https://trac.sagemath.org/ticket/25833