Merge `cep-devel` into `main`

[jaimergp]

Description

This is the long-lived branch where the CEP-11 work has been implemented. It's essentially a rewrite of menuinst to support the CEP-11 standard. The Windows bits of the legacy menuinst are still around for backwards compatibility.

The plan is to release a v2.0alpha with these changes, and iterate once we have feedback. We have been using this branch internally at napari, spyder and other projects, with great success.

Checklist - did you ...

• [X] Add a file to the news directory (using the template) for the next release's release notes?
• [X] Add / update necessary tests?
• [X] Add / update outdated documentation?

[jaimergp]

@conda/constructor - this is ready for review, now that CEP-11 was approved!

The only changes in this PR, compared to what we had in the branch:

• Adjust CI triggers so they target the main branch only
• Add news
• Update the README (including a rename from .rst to .md)

[jaimergp]

Thanks @jezdez! Given how the permissions are structured in this repo and the fact that there are non-existing required tests, I can't merge it yet 😬 Can you take care of the merge with your mighty admin permissions? 🙏

[jezdez]

@jaimergp Fixed that branch protection issue for you and upped the team permissions to admin. Can you commit merge now?