Add "source" button in documentation to view code

The reference documentation https://apple.github.io/pfl-research/ should have links to source code for each component. Useful for navigation and to more effectively copy implemented algorithms and customize them for research.
We use Furo frontend sphinx theme https://github.com/pradyunsg/furo.

(65729691)

Start with the Sphinx documentation configuration for the reference site and review how the Furo theme exposes source links. Verify the result on the published documentation so each component has a usable link to its source code.

Written by the indexing model from the issue text.