Base: Move GML documentation into subsection

That was the whole point of this endeavour :^)

Base/usr/share/man/man5/GML/Widget-SpinBox.md

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+## Name
+
+GML Spin Box Widget
+
+## Description
+
+Defines a GUI spin box widget. This is a number input with buttons to increment and decrement the value.
+
+## Synopsis
+
+`@GUI::SpinBox`
+
+## Examples
+
+```gml
+@GUI::SpinBox {
+    name: "thickness_spinbox"
+    min: 0
+    max: 2
+}
+```
+
+## Registered Properties
+
+| Property | Type  | Possible values    | Description                                  |
+|----------|-------|--------------------|----------------------------------------------|
+| min      | int   | Any 64 bit integer | Minimum number the spin box can increment to |
+| max      | int   | Any 64 bit integer | Maximum number the spin box can increment to |