Code reference requires archive page to display by file

[siobhan]

Code reference users should be able to browse the code reference to view the docs contained in a specific file.

This should follow the URL structure per #324:

developer.wordpress.org/reference/source_files/
developer.wordpress.org/reference/source_files/file-name

We may want to surface this in some way on the code reference landing page so that people can browse/search through file names.

[siobhan]

#173 was marked as a duplicate.