Update index.rst (#391)

* Update index.rst

docs/source/index.rst

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 ==================
 Welcome to SPH-EXA
 ==================
+SPH-EXA is a C++20 simulation code for hydrodynamics simulations (with gravity
+and other physics), parallelized with MPI, OpenMP, CUDA, and HIP.
 
-SPH-EXA is ...
+SPH-EXA is built with high performance, scalability, portability, and
+resilience in mind. Its SPH implementation is based on SPHYNX, ChaNGa, and
+SPH-flow, three SPH codes selected in the PASC SPH-EXA project to act as parent
+and reference codes to SPH-EXA.
+
+The performance of standard codes is negatively impacted by factors such as
+imbalanced multi-scale physics, individual time-stepping, halos exchange, and
+long-range forces. Therefore, the goal is to extrapolate common basic SPH
+features, and consolidate them in a fully optimized, Exascale-ready, MPI+X, SPH
+code: SPH-EXA.
 
 Publications
 ============