Base: Move GML documentation into subsection

That was the whole point of this endeavour :^)

Base/usr/share/man/man5/GML/Usage.md

@@ -0,0 +1,48 @@
+## Name
+
+GML Usage
+
+## Description
+
+How to use GML in SerenityOS C++ applications
+
+## CMake
+
+Include `compile_gml()` your applications CMake file. The header file name and GML string name are not fixed but must follow this convention.
+
+```cmake
+compile_gml(MyApp.gml MyAppGML.h my_app_gml)
+```
+
+Include the name of the header file that will be compiled from your GML file in your `SOURCES`.
+
+```cmake
+set(SOURCES
+    MyAppGML.h
+)
+```
+
+## C++
+
+You can then reference the variable you set (e.g. `my_app_gml`) in your main C++ file using `load_from_gml()`.
+
+```cpp
+load_from_gml(my_app_gml);
+```
+
+From there, you can use `find_descendant_of_type_named` to select widgets from your GML by their `name` property.
+
+```gml
+@GUI::Button {
+    name: "mem_add_button"
+    text: "M+"
+    fixed_width: 35
+    fixed_height: 28
+    foreground_color: "red"
+}
+```
+Is referenced using...
+```cpp
+load_from_gml(calculator_gml);
+m_mem_add_button = *find_descendant_of_type_named<GUI::Button>("mem_add_button");
+```