Docs: remove final mentions of moved_mr_sidebar flag

What does this MR do?

I noticed recently that the moved_mr_sidebar flag was removed in Removes moved_mr_sidebar flag (!127001 - merged) but I didn't hear about it in time. Let's clean up the last few mentions in the docs.

I did a few small / obvious bits of tidying while I was here, but I really just want to take down this mental post-it note in my head that's been lurking around for months.

Related issues

• Related to Removes moved_mr_sidebar flag (!127001 - merged) where the flag was finally removed
• Related to [Feature flag] Enable moved_mr_sidebar globally (#385460 - closed) where the work was planned and executed
• Related to Move notifications and todos to header buttons ... (!132678 - merged)

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