How do I include a subdirectory and its content to the Sphinx output/build directory?
I just came across this question, and can answer for Sphinx 3.3 with Python 3.8.
All I had to do was put html_extra_path = ['extra'] into conf.py. Then I made docs/extra/snippets. Now snippets and all of its content are copied into _build/html/snippets.
I have the following directory structure for my Sphinx project.

root


build
source


index.md
snippets



I want to copy root/source/snippets to build/html/snippets as is; this directory has code snippet files.
I don't think .. include::, :download: and html_extra_path are what I need. Is there a way to do this using the build configuration? Or is the only way to modify the make build scripts.
I am using Python v2.7.13 and Sphinx v1.5.1.