Improve docs syntax checking

Move the checks out of the Makefile into a perl script that can be
called from both the Makefile and meson.build. The set of files checked
is simplified, so it is just all the sgml and xsl files found in
docs/src/sgml directory tree.

Along the way make some adjustments to .cirrus.tasks.yml to support this
better in CI.

Also ensure that the checks are part of the Makefile's html target.

Author: Nazir Bilal Yavuz <byavuz81@gmail.com>
Co-Author: Andrew Dunstan <andrew@dunslane.net>

Discussion: https://postgr.es/m/CAN55FZ3BnM+0twT-ZWL8As9oBEte_b+SBU==cz6Hk8JUCM_5Wg@mail.gmail.com

doc/src/sgml/meson.build

@@ -306,3 +306,26 @@ endif
 if alldocs.length() != 0
   alias_target('alldocs', alldocs)
 endif
+
+sgml_syntax_check = files(
+  'sgml_syntax_check.pl'
+)
+
+test(
+  'sgml_syntax_check',
+  perl,
+  protocol: 'exitcode',
+  suite: 'doc',
+  args: [
+    sgml_syntax_check,
+    '--xmllint',
+      '@0@ --nonet'.format(xmllint_bin.full_path()),
+    '--srcdir',
+      meson.current_source_dir(),
+    '--builddir',
+      meson.current_build_dir(),
+  ],
+  depends: doc_generated
+)
+
+testprep_targets += doc_generated