install ddns example configurations

(I also fixed docs in different commit to
fix pointer to examples in installed location instead of source tree.)

Okayed ("seems ok") via jabber.

doc/Makefile.am

@@ -6,6 +6,8 @@ nobase_dist_doc_DATA  = examples/kea4/single-subnet.json
 nobase_dist_doc_DATA += examples/kea4/several-subnets.json
 nobase_dist_doc_DATA += examples/kea6/simple.json
 nobase_dist_doc_DATA += examples/kea6/several-subnets.json
+nobase_dist_doc_DATA += examples/ddns/sample1.json
+nobase_dist_doc_DATA += examples/ddns/template.json
 
 devel:
 	mkdir -p html