[mlir] NFC: Fix broken links in docs

Summary: This commit fixes missing links that are caused by the repository movement.

Reviewers: Jim, rriddle, jpienaar

Reviewed By: Jim, rriddle, jpienaar

Subscribers: arpith-jacob, mehdi_amini, rriddle, jpienaar, burmako, shauheen, antiagainst, nicolasvasilache, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D72065