doc: add link collection

doc/links.txt

+ulo-storage
+
+    https://gl.kwarc.info/supervision/schaertl_andreas.git
+
+ulo
+
+    https://kwarc.info/people/frabe/Research/CKMRSW_ulo_19.pdf
+
+tetrapodal search
+
+    https://kwarc.info/people/mkohlhase/papers/tetrasearch.pdf
+
+rdf4j
+
+    https://rdf4j.org
+
+mmt
+
+    https://kwarc.info/systems/mmt/