docs: Convert getting_started to markdown

As part of the conversion, give up on including
sources files from the examples directory, and
instead include the content directly. All include
mechanisms add complications. They were already
complicated with xml, and markdown is not making
things easier.

We already did that for fragments, and if you
make changes to these example sources, you
probably need to revise the surrounding text
anyway.

docs/reference/gtk/gtk4-docs.xml

@@ -21,7 +21,7 @@
   <part id="gtk">
     <title>Introduction</title>
     <xi:include href="overview.xml"/>
-    <xi:include href="xml/getting_started.xml"/>
+    <xi:include href="getting_started.xml"/>
     <xi:include href="resources.xml" />
     <xi:include href="xml/question_index.xml" />
   </part>

docs/reference/gtk/meson.build

@@ -373,6 +373,7 @@ expand_content_files = [
 ]
 
 expand_content_md_files = [
+  'getting_started.md',
   'building.md',
   'compiling.md',
   'running.md',
@@ -400,7 +401,6 @@ endif
 
 if get_option('gtk_doc')
   configure_file(input: 'version.xml.in', output: 'version.xml', configuration: version_conf)
-  configure_file(input: 'getting_started.xml.in', output: 'getting_started.xml', configuration: src_dir_conf)
 
   # gtk-markdown-to-docbook uses pandoc
   pandoc = find_program('pandoc', required: true)