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/index.rst

@@ -0,0 +1,22 @@
+PyLDNS documentation
+=======================================
+
+PyLDNS provides an `LDNS`_ wrapper (Python extension module) - the thinnest layer over the library possible.  Everything you can do from the C API, you can do from Python, but with less effort. The purpose of porting LDNS library to Python is to simplify DNS programming and usage of LDNS, however, still preserve the performance of this library as the speed represents the main benefit of LDNS. The proposed object approach allows the users to be concentrated at the essential part of application only and don't bother with deallocation of objects and so on.
+
+.. _LDNS: http://www.nlnetlabs.nl/projects/ldns/
+
+Contents
+----------
+.. toctree::
+	:maxdepth: 2
+
+	install.rst
+	examples/index.rst
+	modules/ldns
+
+Indices and tables
+-------------------
+
+* :ref:`genindex`
+* :ref:`search`
+