fix: populate ldns submodule and add autotools to LDNS build stage

- Re-cloned zonemaster-ldns with --recurse-submodules so the bundled
  ldns C library source (including Changelog and configure.ac) is present
- Added autoconf, automake, libtool to Dockerfile.backend ldns-build stage
  so libtoolize + autoreconf can generate ldns/configure during make

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

zonemaster-ldns/ldns/contrib/ldnsx/source/api/ldnsx.rst

@@ -0,0 +1,15 @@
+LDNSX API Reference
+===================
+
+..	automodule:: ldnsx
+	:members: query, get_rrs, secure_query
+
+Classes
+-------
+.. toctree::
+	:maxdepth: 1
+	:glob:
+
+	resolver
+	packet
+	resource_record