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/index.rst

@@ -0,0 +1,57 @@
+Welcome to ldnsx's documentation!
+=================================
+
+LDNSX: Easy DNS (including DNSSEC) via ldns.
+
+ldns is a great library. It is a powerful tool for
+working with DNS. python-ldns it is a straight up clone of the C
+interface, however that is not a very good interface for python. Its
+documentation is incomplete and some functions don't work as
+described. And some objects don't have a full python API.
+
+ldnsx aims to fix this. It wraps around the ldns python bindings,
+working around its limitations and providing a well-documented, more
+pythonistic interface.
+
+Reference
+=========
+
+.. toctree::
+   :maxdepth: 1
+   
+   api/ldnsx
+
+.. toctree::
+   :maxdepth: 2
+
+   api/resolver
+   api/packet
+   api/resource_record
+
+Examples
+========
+
+Examples translated from ldns examples:
+
+.. toctree::
+   :maxdepth: 1
+
+   examples/ldnsx-axfr
+   examples/ldnsx-dnssec
+   examples/ldnsx-mx1
+   examples/ldnsx-mx2
+
+Others:
+
+.. toctree::
+   :maxdepth: 1
+
+   examples/ldnsx-walk
+
+
+Indices and tables
+==================
+
+* :ref:`genindex`
+* :ref:`search`
+