[mlir][doc] Fix links and references in documentation of Rationale

This patch is the second in a series of patches fixing markdown links and references inside the mlir documentation.

This patch addresses all broken references to other markdown files and sections inside the Rationale folder.

In addition to fixing the links and references like in the previous patch, I also changed references which are URLs to the mlir.llvm.org/docs website, to proper relative markdown references instead.

Differential Revision: https://reviews.llvm.org/D103013