Added apt notes to the knowledge base

knowledge base/linux/README.md

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+# Linux-releated knowledge base
+
+The documents contained here can only (or mostly) be used on GNU/Linux systems.

knowledge base/linux/apt.md

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+# The APT package manager
+
+## TL;DR
+
+```shell
+# mark all packages as non-explicitly installed
+apt-mark auto $(sudo apt-mark showmanual)
+
+# remove orphaned packages
+apt autoremove --purge
+```
+
+## Further readings
+
+- [Apt configuration]
+- [Configuring Apt sources]
+
+[apt configuration]: https://wiki.debian.org/AptConfiguration
+[configuring apt sources]: https://wiki.debian.org/SourcesList