Base: Move GML documentation into subsection

That was the whole point of this endeavour :^)

Base/usr/share/man/man5/GML/Define-widget.md

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+## Name
+
+Library or Application Defined Widgets
+
+## Description
+
+Some applications and libraries find it useful to define their own widgets.
+
+This is done using `REGISTER_WIDGET()`, just as in **LibGUI**. The syntax of the macro is as follows:
+
+```cpp
+REGISTER_WIDGET(namespace, class_name)
+```
+
+This means that every registered widget has to be placed in a namespace. For applications that usually do not need their own namespace, a common approach is to use the application name as the namespace for these registered widgets.
+
+Note that registered widgets need to be constructible without any arguments.
+
+## Examples
+
+```gml
+@Web::OutOfProcessWebView {
+    name: "web_view"
+    min_width: 340
+    min_height: 160
+    visible: false
+}
+```
+
+```cpp
+// OutOfProcessWebView.cpp
+
+REGISTER_WIDGET(Web, OutOfProcessWebView)
+
+...
+
+OutOfProcessWebView::OutOfProcessWebView()
+{
+    set_should_hide_unnecessary_scrollbars(true);
+    set_focus_policy(GUI::FocusPolicy::StrongFocus);
+
+    create_client();
+}
+
+...
+```
+
+## See also
+
+-   [GML Define property(5)](help://man/5/GML/Define-property)