chore: added cron article to the kb

knowledge base/cron.md

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+# Title
+
+## Table of contents <!-- omit in toc -->
+
+1. [TL:DR](#tldr)
+1. [Further readings](#further-readings)
+1. [Sources](#sources)
+
+## TL:DR
+
+Files in `/etc/cron.hourly` and similar need to:
+
+- be executable,
+- match the Debian cron script namespace (`^[a-zA-Z0-9_-]+$`, so script **with an extension** won't work).
+
+```sh
+# Print the names of the scripts which would be invoked.
+sudo run-parts --report --test '/etc/cron.hourly'
+```
+
+## Further readings
+
+## Sources
+
+All the references in the [further readings] section, plus the following:
+
+- [Function of /etc/cron.hourly]
+
+<!-- project's references -->
+
+<!-- internal references -->
+[further readings]: #further-readings
+
+<!-- external references -->
+[Function of /etc/cron.hourly]: https://askubuntu.com/questions/7676/function-of-etc-cron-hourly#607974