[meta] Release preparation steps for 10.1.0

Documenting the upcoming May 22 update to CKEditor 5, which I've already discussed with @catch.

Just moved completed items to their own heading.

Moving the ckeditor5 update to done too: 📌 Update CKEditor 5 to 38.0.1 Fixed .

It's released. :)

Shall we make an issue for 10.2?

Saving credits. Anyone else who helped with the release notes should also be credited.

Automatically closed - issue fixed for 2 weeks with no activity.

Update version to the branch this applies to.