chore: improved folder structure

knowledge base/gnu userland/chmod.md

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+# Chmod
+
+## TL;DR
+
+```sh
+# Copy permissions from another file.
+chmod --reference 'reference-file' 'changed-file'
+```
+
+## Further readings
+
+- [`chmod`][chmod]
+
+## Sources
+
+All the references in the [further readings] section, plus the following:
+
+- [18 selected super-useful Linux one-liners]
+
+<!--
+  References
+  -->
+
+<!-- In-article sections -->
+[further readings]: #further-readings
+
+<!-- Knowledge base -->
+[chmod]: ../chmod.md
+
+<!-- Others -->
+[18 selected super-useful linux one-liners]: https://medium.com/codex/18-selected-super-useful-linux-one-liners-398ba6d20f8c