Lang team process docs have both duplicated bits w/ lang-team and also disjoint bits

E.g. both - https://forge.rust-lang.org/lang/stabilization-procedure.html and - https://lang-team.rust-lang.org/how_to/stabilize.html points to rustc-dev-guide <https://rustc-dev-guide.rust-lang.org/stabilization_guide.html>. Then the [RFC Merge Procedure](https://forge.rust-lang.org/lang/rfc-merge-procedure.html) has overlapping bits with <https://lang-team.rust-lang.org/how_to/propose.html> AFAICT. Some thoughts: - The stabilization template (<https://github.com/rust-lang/rustc-dev-guide/pull/2219>) should eventually move next to stabilization procedure. IMO both should live in *Forge* (rustc-dev-guide should *backlink* to *Forge* because it's a process doc). - Maybe move lang-team docs into the Forge here? cf. the All Hands discussion [#council > who maintains cross-team documentation? @ 💬](https://rust-lang.zulipchat.com/#narrow/channel/392734-council/topic/who.20maintains.20cross-team.20documentation.3F/near/519193654), where one of the ideas was to use Forge for more team process docs.[^help] cc @traviscross [^help]: I'd be happy to help with that migration, if lang is on-board.