Start documenting keyboard shortcuts

I'm sure I don't know all of them.

Signed-off-by: Paul Tötterman <paul.totterman@iki.fi>

docs/shortcuts.md

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+# Keyboard Shortcuts
+
+- <kbd>Ctrl</kbd>+<kbd>m</kbd> - toggle markdown
+- <kbd>Ctrl</kbd>+<kbd>d</kbd> - toggle mic mute
+- <kbd>Ctrl</kbd>+<kbd>e</kbd> - toggle video on/off
+- <kbd>Ctrl</kbd>+<kbd>k</kbd> - jump to named room
+- <kbd>↑</kbd>/<kbd>↓</kbd> - navigate within room list
+- <kbd>PageUp</kbd>/<kbd>PageDown</kbd> - scroll timeline up/down
+- <kbd>Ctrl</kbd>+<kbd>Home</kbd>/<kbd>End</kbd> - jump to timeline start/end