Extend Matfree's documentation

Write a new tutorial:

To add a new tutorial, create a Python file in tutorials/ and fill it with content. Use docstrings (mirror the style in the existing tutorials). Make sure to satisfy the formatting- and linting-requirements. That's all.

Then, the documentation pipeline will automatically convert those into a format compatible with Jupytext, which is subsequently included in the documentation. If you do not want to make the tutorial part of the documentation, make the filename have a leading underscore.

Extend the developer documentation:

To extend the developer documentation, create a new section in the README. Use a second-level header (which is a header starting with "##") and fill the section with content. Then, the documentation pipeline will turn this section into a page in the developer documentation.

Create a new module:

To make a new module appear in the documentation, create the new module in matfree/, and fill it with content. Unless the module name starts with an underscore or is placed in the backend, the documentation pipeline will take care of the rest.