Jupyter supports ESC + M to enter markdown mode. We should support this too. It could just type in the #%% [markdown] for you.
Alternatively we could use it to switch the input field into 'markdown' mode and then it will display the code as markdown. This is what Jupyter does. Might be nice because you can see the markdown as you type it.
Actually we might want to think about all of these:

Validated. On the assumption that this is just talking about the new notebook editor. Does not apply to cells inside the source code.
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Actually we might want to think about all of these: