Last nav cleanup of mainconcepts and sidebar
Last nav cleanup of mainconcepts and sidebar

Summary:
This is hopefully my last major reshuffle of the sidebar and makes it so Main Concepts is mostly complete. There's no longer a
'Coordinating State and Actions' section (yay!) and everything that was there has a new home.

I kinda want to merge Keys and Component Identity into Hooks and State, though may leave as is. The side bar is now structured to be

Intro
Tutorial
Main Concepts
Additional APIs (e.g. animations)
Advanced Content (e.g. migration, codegen)

Reviewed By: SidharthGuglani

Differential Revision: D34788505

fbshipit-source-id: d0f94bd84ce9543d66933d504b737185f68d1a5b

facebook/litho,facebook/litho,facebook/litho,facebook/litho,facebook/litho,facebook/litho