Auto merge of #119024 - lqd:linker-docs, r=petrochenkov

Document unstable linker flags in the unstable book

This PR:
- removes the unstable linker flavors from the stable documentation
- documents the unstable `-C linker-flavor` values in the unstable book (including the ones previously described as stable)
- documents the unstable `-C link-self-contained` components in the unstable book

I don't know if these have some common reviewers, but if not r? `@petrochenkov` as the most knowledgeable person in this area. Please feel free to reassign, I know that you are quite busy.

Author: bors