Write some user documentation

[schuderer]

DoD: good-enough description of

• how to begin
• why did we make it, what does it solve
• how to use it (best)
• what do I need to ACTUALLY make use of it in production.

[schuderer]

Basic intro done in e3ae765

Still more documentation needed (guides on how to write models, configure datasources, extend datasources, documentation of functionality (generated?))

[schuderer]

Detailed usage description should go into the file docs/usage.rst, which is currently empty.