[mlir][Linalg][doc] Add Design Document for the Linalg Dialect

Summary: This revision adds a Rationale for the Linalg Dialect

Reviewers: rriddle, mehdi_amini, ftynse, albertcohen

Reviewed By: albertcohen

Subscribers: merge_guards_bot, jfb, jpienaar, burmako, shauheen, antiagainst, arpith-jacob, mgester, lucyrfox, aartbik, liufengdb, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D73595