[MLIR][spirv] Fix links in docs after repo migration

Summary:
This commit updates links to SPIR-V dialect code to LLVM monorepo
on GitHub. It also points to the operation doc on mlir.llvm.org.

Reviewers: mravishankar, denis13, ftynse

Reviewed By: ftynse

Subscribers: merge_guards_bot, mehdi_amini, rriddle, jpienaar, burmako, shauheen, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D71926