Add docs files

docs/gthree-gtk3-docs.xml

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+<?xml version="1.0"?>
+<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.3//EN"
+                      "http://www.oasis-open.org/docbook/xml/4.3/docbookx.dtd" [
+  <!ENTITY % local.common.attrib "xmlns:xi  CDATA  #FIXED 'http://www.w3.org/2003/XInclude'">
+]>
+
+<book id="index" xmlns:xi="http://www.w3.org/2003/XInclude">
+  <bookinfo>
+    <title>Gthree Reference Manual</title>
+    <releaseinfo>
+      <para>This document is the API reference for Gthree Gtk3.</para>
+      <para>
+        If you find any issues in the API reference, please report it using
+        <ulink type="http" url="https://github.com/alexlarsson/gthree/issues">the
+        online bug reporting tool</ulink>.
+      </para>
+    </releaseinfo>
+  </bookinfo>
+
+  <part id="widget">
+    <title>Widget</title>
+
+    <xi:include href="xml/gthreearea.xml" />
+  </part>
+
+</book>

docs/gthree-gtk3-sections.txt

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+<SECTION>
+<FILE>gthreearea</FILE>
+GthreeArea
+GthreeAreaClass
+<SUBSECTION>
+gthree_area_new
+gthree_area_set_camera
+gthree_area_get_camera
+gthree_area_set_scene
+gthree_area_get_scene
+gthree_area_get_renderer
+<SUBSECTION Standard>
+GTHREE_AREA
+GTHREE_IS_AREA
+GTHREE_TYPE_AREA
+gthree_area_get_type
+</SECTION>