UX for switching between Markdown or Content Editor

Problem

There have been many different small discussion about the new feature to switch between Markdown and Content Editor, which have led to an unclear vision around where the switcher should be visible.

Proposal

Before we roll this out to users, we should establish a few core questions:

• Should this be a user preference, or should it always be visible on every Editor instance?
• Should each Editor instance use the Content Editor?
• When can the new Content Editor be rolled out to users more widely?
• How will we make users aware of this new feature and onboard them?

Decision on the language

In !120850 (comment 1427557955) it was decided what to call the two modes of the editor:

• Plain Text Editor (previously: Markdown Editor)
• Rich Text Editor (previously: Content Editor)

MR to update docs references: !124746 (merged)