[Developer] Integrate Repo Docs into the Reference Area

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Developer content may come from docs that live in the relevant repo. Update the reference space so that you can easily navigate to documentation for repos that are a part of the Open edX platform.

• Do this work ourselves on 2-3 pilot repos.
• Use that to write how-tos that can be used by maintainers to do this work on their own repos.
• Ask the maintainers to do this work in repos they maintain, give them a quarter or two to complete it.

[feanil]

This might be useful for making the site seem more unified: https://web.archive.org/web/20200805215833/http://www.virtualroadside.com/blog/index.php/2017/01/14/integrated-subproject-sites-on-readthedocs-org/

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#89 landed which adds one repo and shows the pattern we want to use for now.