Fixed build doc

Due to an update of sphinxcontrib-bibtex a .bib file must be specified for the documentation to work.

doc/bibtex.json

+{
+    "cited": {
+        "sphinx/forcemodels": [
+            "luo1993lattice",
+            "guo2002discrete",
+            "schiller2008thermal",
+            "buick2000gravity"
+        ]
+    }
+}
\ No newline at end of file