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/examples/example6.rst

@@ -0,0 +1,12 @@
+Read zone file
+===============================
+
+This example shows how to read the content of a zone file
+
+.. literalinclude:: ../../../examples/ldns-zone.py
+   :language: python
+
+Zone file ``zone.txt``:
+
+.. literalinclude:: ../../../examples/zone.txt
+