Added flatpak notes to the knowledge base

knowledge base/linux/flatpak.md

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+# Flatpak
+
+## TL;DR
+
+```shell
+# add the `flathub` remote
+flatpak remote-add --if-not-exists flathub https://flathub.org/repo/flathub.flatpakrepo
+flatpak remote-add --user --if-not-exists flathub https://flathub.org/repo/flathub.flatpakrepo
+
+# search for `vscode`
+flatpak search vscode
+
+# install `vscode`
+flatpak install --user flathub com.visualstudio.code-oss
+
+# uninstall unused packages
+flatpak uninstall --unused
+```
+
+## Further readings
+
+- [Using Flatpak] getting started guide on the official [documentation]
+- [How to clean up Flatpak apps to clear disk space]
+
+[documentation]: https://docs.flatpak.org/en/latest/
+[using flatpak]: https://docs.flatpak.org/en/latest/using-flatpak.html
+
+[how to clean up flatpak apps to clear disk space]: https://www.debugpoint.com/2021/10/clean-up-flatpak/