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 https://github.com/rust-lang/rust/pull/90128#issuecomment-948569123). Per the discussion at https://github.com/rust-lang/rust/pull/93661#discussion_r799783363 this adds those specifics as an official description of the format. cc #89917