Extend our current documentation to quickly cover how to operate an ALBS instance

[javihernandez]

I see that, when new potential contributors approach us and try deploying/using their own instance of the build system, the first obstacle they face is that they don't really know what's going on behind the curtains; the services involved, their relationships, mandatory vs optional services, how to check problems, how things run, the containers they live in, how to restart things the right way, etc.

Maybe we should compile a small document covering the topics above and extend our current documentation with this content.