docs: add documention for ravaflow24 mixture model

docs/source/simulator/ravaflow24.rst

 Ravaflow24 Mixture Model
 ========================
 
+`r.avaflow 2.4` is a GIS-supported open source software for mass flow modeling.
+It is developed by :cite:t:`Mergili2017`. For more information see its official
+user manual `here <https://www.landslidemodels.org/r.avaflow/>`_. 
+
+In :py:mod:`PSimPy.simulator.ravaflow24`, we have implemented class 
+:class:`.Ravaflow24Mixture`. It provides an Python interface to directly run
+the `Voellmy-tpye shallow flow model` of `r.avaflow 2.4` from within Python.
+For detailed theory of `Voellmy-tpye shallow flow model`, one can refer to
+:cite:t:`Christen2010` and :cite:t:`Fischer2012`.
+
 Ravaflow24Mixture Class
 -----------------------