Add example for learning capabilities with torchpme (#104)

* Add example for learning capabilities with torchpme and environment configuration
---------

Co-authored-by: Davide Tisi <[email protected]>

Author: E-Rum

examples/torchpme/README.rst

@@ -0,0 +1,6 @@
+Learning Capabilities with torchpme
+=======================================
+
+This example demonstrates the capabilities of the `torchpme` package, focusing on
+learning target charges and utilizing the :class:`CombinedPotential` class to evaluate
+potentials that combine multiple pairwise interactions with optimizable ``weights``.