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 formatter and linter. That's all.

Then, the documentation pipeline will automatically convert those into a format compatible with Jupytext, which subsequently includes it into 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 (a header that starts 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,