Add sphinx

[Guillermo-Mijares-Vilarino]

Summary

Set up a Sphinx-based documentation page for this project.

Details and comments

First sphinx-quickstart was used to generate the basic framework. After that, a page for each type of documentation was created and included in the sidebar via a toctree in docs/index.rst.

For the API Reference, some changes were made to the docstrings in the corresponding __init__.py to accommodate the html pages.

Needed installs:

• sphinx
• qiskit_sphinx_theme
• nbsphinx (Sphinx extension that enables to use Jupyter notebooks in documentation)