games, synthetis: improve Doxygen

* spot/twaalgos/game.hh, spot/twaalgos/synthesis.hh,
spot/twaalgos/aiger.hh: Declare new Doxygen groups for games and
synthesis.
* doc/spot.bib: Cleanup.

doc/spot.bib

@@ -361,11 +361,23 @@
   doi		= {10.1016/S0020-0190(00)00113-7}
 }
 
-@article{finkbeiner2021specification,
-  title={Specification Decomposition for Reactive Synthesis (Full Version)},
-  author={Finkbeiner, Bernd and Geier, Gideon and Passing, Noemi},
-  journal={arXiv preprint arXiv:2103.08459},
-  year={2021}
+@TechReport{	  finkbeiner.21.arxiv,
+  title		= {Specification Decomposition for Reactive Synthesis (Full
+		  Version)},
+  author	= {Bernd Finkbeiner and Gideon Geier and Noemi Passing},
+  year		= 2021,
+  url		= {https://arxiv.org/abs/2103.08459/}
+}
+
+@InProceedings{	  finkbeiner.21.nfm,
+  title		= {Specification Decomposition for Reactive Synthesis},
+  author	= {Bernd Finkbeiner and Gideon Geier and Noemi Passing},
+  booktitle	= {Procedings of NASA Formal Methods (NFM'21)},
+  volume	= {12673},
+  series	= {Lecture Notes in Computer Science},
+  publisher	= {Springer},
+  doi		= {10.1007/978-3-030-76384-8_8},
+  year		= {2021}
 }
 
 @InProceedings{	  gastin.01.cav,