docs: Pass --standalone to pandoc

This flag causes pandoc to emit a proper doctype
declaration and, crucially, namespace declarations
for the xlink namespace that it insists on using
for href attributes. Without this, putting external
links in md documents doesn't survive the journey
through xml.

docs/reference/gtk/gtk-markdown-to-docbook

@@ -177,6 +177,7 @@ def ConvertToDocbook(infile, outfile):
     subprocess.check_call(["pandoc", infile, "-o", outfile,
                            "--from=" + input_format,
                            "--to=" + output_format,
+                           "--standalone",
                            "--top-level-division=" + division])
 
 def ExpandGtkDocAbbreviations(infile, outfile):