gtk-launch: add page (#15629)

Co-authored-by: Lena <126529524+acuteenvy@users.noreply.github.com>

pages/linux/gtk-launch.md

@@ -0,0 +1,8 @@
+# gtk-launch
+
+> Launch applications from `.desktop` files that reside in standard locations.
+> More information: <https://manned.org/gtk-launch>.
+
+- Launch an application:
+
+`gtk-launch {{Application_name}}`