dm-tool: add page (#7710)

pages/linux/dm-tool.md

@@ -0,0 +1,20 @@
+# dm-tool
+
+> A tool to communicate with the display manager.
+> More information: <https://manned.org/dm-tool>.
+
+- Show the greeter while keeping current desktop session open and waiting to be restored upon authentication by logged in user:
+
+`dm-tool switch-to-greeter`
+
+- Lock the current session:
+
+`dm-tool lock`
+
+- Swicth to a specific user, showing an authentication prompt if required:
+
+`dm-tool switch-to-user {{username}} {{session}}`
+
+- Add a dynamic seat from within a running LightDM session:
+
+`dm-tool add-seat {{xlocal}} {{name}}={{value}}`