Added crontab's notes to the KB

knowledge base/crontab.md

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+# Crontab
+
+## TL;DR
+
+```sh
+# List existing jobs.
+crontab -l
+sudo crontab -l -u other_user
+
+# Edit crontab files.
+crontab -e
+sudo crontab -e -u other_user
+
+# Replace the current crontab with the contents of a given file.
+crontab path/to/file
+sudo crontab -u other_user path/to/file
+
+# Remove all cron jobs.
+crontab -r
+sudo crontab -r -u other_user
+```
+
+```text
+# Run 'pwd' every day at 10PM.
+0 22 * * * pwd
+
+# Run 'ls' every 10 minutes.
+*/10 * * * * ls
+
+# Run a script at 02:34 every Friday.
+34 2 * * Fri /absolute/path/to/script.sh
+```
+
+## Sources
+
+- [cheat.sh]
+
+[cheat.sh]: https://cheat.sh/crontab