Use matplotlib sphinx extensions to build documentation

We should have a few simple examples in the documentation, generated inline with the matplotlib plotting extensions for sphinx.

For example, we can use this to generate one of the main figures:

.. plot::

   import matplotlib.pyplot as plt
   from grid_strategy.strategies import SquareStrategy

   specs = SquareStrategy('center').get_grid(7)

   for i, spec in enumerate(specs):
      ax = plt.subplot(spec)

      ax.text(0.5, 0.5, f"Subplot: {i}", color='white',
             fontweight='bold', va="center", ha="center")
      ax.tick_params(axis='both', bottom=False, top=False, left=False,
                    right=False, labelbottom=False, labelleft=False)

      ax.patch.set_facecolor('#2E2E2E')

   plt.show()

Locate the Sphinx documentation configuration and documentation entry point, then use the shown SquareStrategy example as the starting case. Done means the selected examples build as inline Matplotlib plots in the generated documentation.