Add `alt` tag to every image

[Nearoo]

The alt tag (short for "alternative") describes the content of the image in text form. How to add an alt tag:

If you see an image like this:

.. |/images/hei_operator2.png| image:: /images/hei_operator2.png

Check what the image shows, and add an alt tag like this:

.. |/images/hei_operator2.png| image:: /images/hei_operator2.png
    :alt: Heisenberg's Operator & Mod Matrix with labels that explain different features