umount: split common and util-linux implementations (#12045)

* umount: split common and util-linux implementations

* umount: remove additional newlines

pages/linux/umount.md

@@ -0,0 +1,25 @@
+# umount
+
+> Unlink a filesystem from its mount point, making it no longer accessible.
+> A filesystem cannot be unmounted when it is busy.
+> More information: <https://manned.org/umount.8>.
+
+- Unmount a filesystem, by passing the path to the source it is mounted from:
+
+`umount {{path/to/device_file}}`
+
+- Unmount a filesystem, by passing the path to the target where it is mounted:
+
+`umount {{path/to/mounted_directory}}`
+
+- When an unmount fails, try to remount the filesystem read-only:
+
+`umount --read-only {{path/to/mounted_directory}}`
+
+- Recursively unmount each specified directory:
+
+`umount --recursive {{path/to/mounted_directory}}`
+
+- Unmount all mounted filesystems (except the `proc` filesystem):
+
+`umount -a`