Support for public files on output docs site

[brockfanning]

Testing instructions: In the data repository, create a folder called public and add any files inside that folder (such as a robots.txt file). These files should appear in the generated output docs site.

Q	A
Feature branch/test site URL	[Link](add link here)
Fixed issues	Fixes https://github.com/open-sdg/open-sdg/issues/2008
Related version	2.3.0-dev
Bugfix, feature or docs?	Feature

• [ ] Keeps backward-compatibility
• [ ] Includes tests
• [ ] Updated docs
• [ ] Updated config forms
• [ ] Added CHANGELOG entry