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/examples/ldnsx-dnssec.rst

@@ -0,0 +1,6 @@
+DNSSEC Example
+==============
+
+.. literalinclude:: ../../examples/ldnsx-dnssec.py
+	:language: python
+	:linenos: