Add keyboard shortcut for checklist

[schiessle]

Checklists are the most common things I use, especially in collectives or notes.

It would be great to have a keyboard shortcut for it. If I can decide I would love to have "Ctrl+l" because it is the same used by other famous not taking tools like Obsidian.

[juliusknorr]

Actually we have one alrady (Ctrl-shift-9) however it is only shown in the tooltip of the menubar icon, not in the help modal 🙈 We should fix that.

Another quick way would be to just type markdown checklists - [ ]