Move to phase 4: Stablization

[mflatt]

Rhombus is now complete and stable enough to be useful for early adopters, and there's now a Rhombus web page at https://rhombus-lang.org/.

I recommend that we formally move to the next phase 4 of the Rhombus plan, but rename that to phase "Stabilization" (as suggested by @samth) and make explicit the policy on breaking changes at each phase (also suggested by @samth). Especially to clarify the long-term breaking-change policy, we can add "Production" as a new phase that formalizes the end of the original plan.

The proposed changes are in #633.

[LiberalArtist]

In the section on what will happen to Racket, “beyond phase 4” should presumably be updated to “beyond phase 5.”

I don't think it's directly relevant to this change, but expanding that section might also be valuable, since people seem to have persistent questions and confusion about it.

[mflatt]

Updated. Yes, that section had become stale, so I'm proposing a revision there via the PR.

[mflatt]

Thanks for the help updating the document (here and in the PR). We seem to all be in agreement about moving the next stage, so I'll merge, and we're officially in phase 4!