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/equations.rst

@@ -0,0 +1,42 @@
+Equations
+==========
+Equations are used in PINA to make easy the training. During problem definition
+each `equation` passed to a `Condition` object must be an `Equation` or `SystemEquation`.
+An `Equation` is simply a wrapper over callable python functions, while `SystemEquation` is
+a wrapper arounf a list of callable python functions. We provide a wide rage of already implemented
+equations to ease the code writing, such as `FixedValue`, `Laplace`, and many more.
+
+
+.. currentmodule:: pina.equation.equation_interface
+.. autoclass:: EquationInterface
+   :members:
+   :show-inheritance:
+
+.. currentmodule:: pina.equation.equation
+.. autoclass:: Equation
+   :members:
+   :show-inheritance:
+
+
+.. currentmodule:: pina.equation.system_equation
+.. autoclass:: SystemEquation
+   :members:
+   :show-inheritance:
+
+
+.. currentmodule:: pina.equation.equation_factory
+.. autoclass:: FixedValue
+   :members:
+   :show-inheritance:
+
+.. autoclass:: FixedGradient
+   :members:
+   :show-inheritance:
+
+.. autoclass:: FixedFlux
+   :members:
+   :show-inheritance:
+
+.. autoclass:: Laplace
+   :members:
+   :show-inheritance: