khal: add page (#4386)

pages/common/khal.md

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+# khal
+
+> A text-based calendar and scheduling application for the command line.
+> More information: <https://lostpackets.de/khal>.
+
+- Start khal on interactive mode:
+
+`ikhal`
+
+- Print all events scheduled in personal calendar for the next seven days:
+
+`khal list -a {{personal}} {{today}} {{7d}}`
+
+- Print all events scheduled not in personal calendar for tomorrow at 10:00:
+
+`khal at -d {{personal}} {{tomorrow}} {{10:00}}`
+
+- Print a calendar with a list of events for the next three months:
+
+`khal calendar`
+
+- Add new event to personal calendar:
+
+`khal new -a {{personal}} {{2020-09-08}} {{18:00}} {{18:30}} "{{Dentist appointment}}"`