Auto merge of #97571 - ehuss:symbol-mangling, r=michaelwoerister

Add documentation on v0 symbol mangling.

This adds official documentation for the v0 symbol mangling format, migrating the documentation from [RFC 2603](https://rust-lang.github.io/rfcs/2603-rust-symbol-name-mangling-v0.html).
The format was originally stabilized as the `-C symbol-mangling-version` option, but the specifics were not stabilized (per rust-lang/rust#90128 (comment)).
Per the discussion at rust-lang/rust#93661 (comment) this adds those specifics as an official description of the format.

cc #89917

Author: bors