Tutorials and Doc (#191)

* Tutorial doc update
* update doc tutorial
* doc not compiling

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Co-authored-by: Dario Coscia <dcoscia@euclide.maths.sissa.it>
Co-authored-by: Dario Coscia <dariocoscia@Dario-Coscia.local>

docs/source/_rst/_tutorials.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**. 
+
+.. toctree::
+    :maxdepth: 3
+    :hidden:
+
+Getting started with PINA
+-------------------------
+   * :doc:`Introduction to PINA for Physics Informed Neural Networks training<tutorials/tutorial1/tutorial>`
+   * :doc:`Building custom geometries with PINA Location class<tutorials/tutorial6/tutorial>`
+
+Physics Informed Neural Networks
+--------------------------------
+   * :doc:`Two dimensional Poisson problem using Extra Features Learning<tutorials/tutorial2/tutorial>`
+   * :doc:`Two dimensional Wave problem with hard constraint<tutorials/tutorial3/tutorial>`
+
+Neural Operator Learning
+------------------------
+   * :doc:`Two dimensional Darcy flow using the Fourier Neural Operator<tutorials/tutorial5/tutorial>`
+
+Supervised Learning
+-------------------
+   * :doc:`Unstructured convolutional autoencoder via continuous convolution<tutorials/tutorial4/tutorial>`