Document Shortcuts

I couldn't find any documentation, so I looked at the [source](https://github.com/chroxify/haptic/blob/main/apps/web/src/lib/components/shared/command-menu/command.svelte#L26).

```
'cmd+k': 'default',
'cmd+j': 'open_note',
'cmd+shift+m': 'move_note',
'cmd+shift+t': 'change_theme',
'cmd+o': 'open_collection',
'cmd+shift+h': 'help_and_feedback',
'cmd+shift+l': 'share'
```

`cmd` is replaced by `ctrl` on Linux (and presumably Windows).

The `default` action is the *command palette*. Pressing `ctrl+k` reveals a ton more functions and shortcuts, which can be found listed in a different [source file](https://github.com/chroxify/haptic/blob/main/apps/web/src/lib/components/shared/command-menu/commands.ts#L35).

Hopefully this helps someone else.