doc: Check for pandoc

Add an explicit check for pandoc, so we get a clear
error message if it is missing.

docs/reference/gtk/meson.build

@@ -403,6 +403,8 @@ 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)
   expand_md = find_program('gtk-markdown-to-docbook')
   expand_md_targets = []
   foreach t : expand_content_md_files