Add extensions/addons section to read the docs

[achiyae]

Add a section that lists all plugins/addons to bpjs.
Debateable if we want to add also documentation for them or just list them.

[michbarsinai]

I think we need to revive b-prog.org, put a standard WordPress there, and maintain the list there (as well as many other things). ReadTheDocs is a high-level technical doc for BPjs, other tools should probably have their documentation in their repos.

Another option is to get BGU to open an OpenScholar app for us, I think they do that now.