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/_rst/_tutorial.rst

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+PINA Tutorials
+==============
+
+In this folder we collect useful tutorials in order to understand the principles and the potential of **PINA**. 
+
+Getting started with PINA
+-------------------------
+.. toctree::
+   :maxdepth: 3
+   :titlesonly:
+
+   Introduction to PINA for Physics Informed Neural Networks training <tutorials/tutorial1/tutorial.rst>
+   Building custom geometries with PINA Location class <tutorials/tutorial6/tutorial.rst>
+
+Physics Informed Neural Networks
+--------------------------------
+.. toctree::
+   :maxdepth: 3
+   :titlesonly:
+
+   Two dimensional Poisson problem using Extra Features Learning<tutorials/tutorial2/tutorial.rst>
+   Two dimensional Wave problem with hard constraint<tutorials/tutorial3/tutorial.rst>
+
+
+Neural Operator Learning
+------------------------
+.. toctree::
+   :maxdepth: 3
+   :titlesonly:
+
+   Two dimensional Darcy flow using the Fourier Neural Operator<tutorials/tutorial5/tutorial.rst>
+
+Supervised Learning
+-------------------
+.. toctree::
+   :maxdepth: 3
+   :titlesonly:
+
+   Unstructured convolutional autoencoder via continuous convolution<tutorials/tutorial4/tutorial.rst>