Documentation for v0.1 version (#199)

* Adding Equations, solving typos
* improve _code.rst
* the team rst and restuctore index.rst
* fixing errors

---------

Co-authored-by: Dario Coscia <dariocoscia@dhcp-015.eduroam.sissa.it>

docs/source/_cite.rst

@@ -0,0 +1,21 @@
+Cite PINA
+==============
+
+If PINA has been significant in your research, and you would like to acknowledge the project in your academic publication,
+we suggest citing the following paper:
+
+*Coscia, D., Ivagnes, A., Demo, N., & Rozza, G. (2023). Physics-Informed Neural networks for Advanced modeling. Journal of Open Source Software, 8(87), 5352.*
+
+Or in BibTex format
+
+.. code:: bash
+
+    @article{coscia2023physics,
+            title={Physics-Informed Neural networks for Advanced modeling},
+            author={Coscia, Dario and Ivagnes, Anna and Demo, Nicola and Rozza, Gianluigi},
+            journal={Journal of Open Source Software},
+            volume={8},
+            number={87},
+            pages={5352},
+            year={2023}
+            }