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/python/docs/source/modules/ldns.rst

@@ -0,0 +1,40 @@
+LDNS module documentation
+================================
+
+Here you can find the documentation of pyLDNS extension module. This module consists of several classes and a couple of functions.
+
+.. toctree::
+	:maxdepth: 1
+	:glob:
+
+	ldns_resolver
+	ldns_pkt
+	ldns_rr
+	ldns_rdf
+	ldns_dname
+	ldns_rr_list
+	ldns_zone
+	ldns_key
+	ldns_key_list
+	ldns_buffer
+	ldns_dnssec
+	ldns_func
+
+
+
+
+**Differences against libLDNS**
+
+* You don't need to use ldns-compare functions, instances can be compared using standard operators <, >, = ::
+	
+	if (some_rr.owner() == another_rr.rdf(1)):
+		pass
+
+* Classes contain static methods that create new instances, the name of these methods starts with the new\_ prefix (e.g. :meth:`ldns.ldns_pkt.new_frm_file`).
+
+* Is it possible to print the content of an object using ``print objinst`` (see :meth:`ldns.ldns_resolver.get_addr_by_name`).
+
+* Classes contain write_to_buffer method that writes the content into buffer.
+
+* All the methods that consume parameter of (const ldns_rdf) type allows to use string instead (see :meth:`ldns.ldns_resolver.query`).
+