at: add page, atq: add page, atrm: add page, and batch: add page (#1344)

commands to queue commands execution at specified time
or when system load allows

pages/common/at.md

@@ -0,0 +1,16 @@
+# at
+
+> Execute commands once at a later time.
+> Service atd (or atrun) should be running for the actual executions.
+
+- Execute commands from standard input in 5 minutes (press `Ctrl + D` after entering commands):
+
+`at now + 5 min`
+
+- Execute a command from standard input at 10:00 AM today:
+
+`echo "{{./make_db_backup.sh}}" | at 1000`
+
+- Execute commands from a given file next Tuesday:
+
+`at -f {{path/to/file}} 9:30 PM Tue`