systemd-cgls: add page (#10933)

Co-authored-by: K.B.Dharun Krishna <kbdharunkrishna@gmail.com>
Co-authored-by: Lena <126529524+acuteenvy@users.noreply.github.com>

pages/linux/systemd-cgls.md

@@ -0,0 +1,16 @@
+# systemd-cgls
+
+> Show the contents of the selected Linux control group hierarchy in a tree.
+> More information: <https://www.freedesktop.org/software/systemd/man/systemd-cgls.html>.
+
+- Display the whole control group hierarchy on your system:
+
+`systemd-cgls`
+
+- Display a control group tree of a specific resource controller:
+
+`systemd-cgls {{cpu|memory|io}}`
+
+- Display the control group hierarchy of one or more systemd units:
+
+`systemd-cgls --unit {{unit1 unit2 ...}}`