Docs: Document state and data approaches with examples

[tyxla]

We've recently been discussing with the rest of @Automattic/team-calypso-frameworks that our state and data approaches are too many, and for someone not familiar with Calypso it's not very clear what to go with.

To improve that, we should create a document where we clearly outline all current approaches and all deprecated ones, plus examples that demonstrate what is suggested to be used in a certain scenario.