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 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? @ 💬, 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.

Compare forge.rust-lang.org/lang/stabilization-procedure.html with lang-team.rust-lang.org/how_to/stabilize.html and rustc-dev-guide.rust-lang.org/stabilization_guide.html, then review the RFC Merge Procedure and lang-team proposal pages for overlap. Coordinate the ownership decision and migration scope; done means duplicated process guidance is consolidated with backlinks and the stabilization template has an agreed home.