doc: add preliminary reference to paper

doc/references.bib

+@inproceedings(ECRTS23-Rheinfels,
+  author     =  {Rheinfels, Tim and Gaukler, Maximilian and Ulbrich, Peter},
+  title      =  {{A New Perspective on Criticality: Efficient State Abstraction and Runtime Monitoring of Mixed-Criticality Real-time Control Systems}},
+  booktitle  =  {35th Euromicro Conference on Real-Time Systems (ECRTS 2023)},
+  year       =  {2023},
+  doi        =  {10.4230/LIPIcs.ECRTS.2023.16},
+  pages      =  {t.b.d.},
+  notes      =  {Accepted},
+}