[PowerPC][docs] Update IBM official docs in Compiler Writers Info page
Just realized that most of the links in this page are deprecated.
So update some important reference here:
Move deprecated ones into "Other documents..".
Reviewers: #powerpc, hfinkel, nemanjai
Reviewed By: hfinkel
Subscribers: shchenz, llvm-commits
Differential Revision: https://reviews.llvm.org/D68817