Improve how docs outside the main docs directory get included

[robrap]

See the following PR for an alternative possibility of using automatically created symlinks, rather than having to create duplicate files, to index other docs in the repository. This PR was closed in draft, but someone might want to take it over the line.

• DO NOT MERGE feat: sphinx plugin to get docs outside the main tree #293

Alternatively, others may have resolved this problem in another way in other repos. Outreach can be done to determine how this is handled elsewhere.