I'd try to summarise key points in every block after the title. E.g. "Useful when xyz"
I'd try to simplify code with pretending static imports (e.g. configurations) where possible
Maybe use SimpleBuilder in examples, and only make an extra note "If you have parameterised builder (link to documentation on that), you could test that the child was created with the right parameter:" + example where you have a test on that. This would simplify code.
When a documentation page have several different topics, I'd consider starting with the simpler ones before the more complex ones, in an increasing order.
I'd reconsider focusing on reactive apis, as they're optional. Maybe in a separate section.
Related to https://github.com/badoo/RIBs/pull/303
Would be nice to have test sample app done by newjoiners.