Sphinx only copies the source files and attachments that are referenced
into the build directory, meaning that if you have additional files you
want to include, you have to copy them manually in the Makefile.

This change introduces a new extension, collect_files, that takes a list
of filenames/patterns and collects those from within the source
directory to the right place in the build directory.

Tested this with some changes in one of our doc bundles. Successfully

collected some files at different levels in the tree.