[mlir][docs] Refactor the layout of the docs folder

Summary:
This revision performs a few refactorings on the main docs folder. Namely it:
* Adds a new Rationale/ folder to contain various rationale documents
* Moves several "getting started" documents to the Tutorials/ folder
* Cleans up the titles of various documents

Differential Revision: https://reviews.llvm.org/D77934