calendar: add page

pages/common/calendar.md

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+# calendar
+
+> Display upcoming events from a calendar file.
+> Basic event format: "(month)/(day)	(Description)" (tab-separated).
+
+- Show events for today and tomorrow (or the weekend on Friday) from the default calendar:
+
+`calendar`
+
+- Look [A]head, showing events for the next 7 days:
+
+`calendar -A 7`
+
+- Look [B]ack, showing events for the previous 7 days:
+
+`calendar -B 7`
+
+- Show events from a custom calendar file:
+
+`calendar -f {{/path/to/calendar_file}}`
+
+- Example entry for Christmas:
+
+`12/25   Christmas`
+
+- Example entry for US Thanksgiving (4th Thursday of November):
+
+`11/ThuFourth    Thanksgiving`
+
+- Example entry for US Memorial Day (last Monday of May):
+
+`May MonLast      Memorial Day`