Edit on GitHub link broken on shared source documents

Bug is as described in the title.
To test: go to https://docs.opendatakit.org/docs-tech-guide
Click Edit on GitHub. -> 404

Open https://docs.opendatakit.org/docs-tech-guide and reproduce the 404 from its Edit on GitHub link. Then inspect the Sphinx documentation setup and the source-document link configuration for shared documents. Done means the link reaches the correct GitHub source page instead of returning 404.

Written by the indexing model from the issue text.