Auto merge of #107099 - edward-shen:edward-shen/rustdoc-remap-path-prefix, r=GuillaumeGomez

rustdoc: Add support for --remap-path-prefix

Adds `--remap-path-prefix` as an unstable option. This is implemented to mimic the behavior of `rustc`'s `--remap-path-prefix`.

This flag similarly takes in two paths, a prefix to replace and a replacement string.

This is useful for build tools (e.g. Buck) other than cargo that can run doc tests.

cc: `@dtolnay`

Author: bors