It would be tremendously fantastibulous to have an up-to-date developers’ guide. Is this part of the BTR WG group mandate? I don’t think it is, but if people want to start working on it I don’t want to block them 
This idea was sparked by this conversation: How do edX developers run edx-platform unit tests?
Also, @Andres.Aulasneo courageously started a Contributor’s guide: https://openedx.atlassian.net/wiki/spaces/COMM/pages/1682211674/Open+edX+contributor+s+guide
Does someone feel like leading an effort to centralize and update the various sources of documentation?