Doc site updates

[apiology]

I'm going to take a pass at the documentation site and add some updates.

Possible changes:

• Talk more about RBS collections and using them to get better gem types
• See if we point folks towards plugins at the right points
• Drop comment about typechecking being at WIP level
• Point to some ways to work your way up to strong typechecking in a current codebase
• Talk about flow-sensitive-typing and how folks can make their code more provably typesafe
• Double check IDE configuration
• Talk about caching and how to use it in CI?