What is the merging policy for stdlib? (contributing.md is nonsense)

[SkySkimmer]

Contributing.md says

Our CODEOWNERS file associates a team of maintainers to each component.

etc, I guess copied from the coq/coq contributing.md, but stdlib has no codeowners file so this whole section is not applicable.

[Zimmi48]

Also related is the discussion that I attempted to start on Zulip: https://coq.zulipchat.com/#narrow/channel/478198-Stdlib-devs-.26-users/topic/Repository.20access.2C.20branch.20protection.20and.20.60CODEOWNERS.60

[Zimmi48]

In today's Rocq Call, we confirmed that self-merging is OK for the stdlib when the author feels like they don't need (more) reviews. We should incrementally update the contributing guide to match the actual practice.