Enable sphinx extensions for docs

[awvwgk]

Remove the toggle CSS in favour of actual sphinx extensions.

• add sphinx_togglebutton to allow adding the toggle CSS class
• add sphinx_copybutton to automatically make codeblocks copyable
• add sphinx_panels for tabbing functionality

[fabothch]

These are useful additions, making it easier to structure program output, make input copying easy.

The usage is described at: https://sphinx-panels.readthedocs.io/en/latest/#installation

To use it on my machine i had to run:

pip install sphinx-panels
pip install sphinx_copybutton
pip install sphinx_togglebutton

[awvwgk]

I'm usually doing all Python stuff with mamba, but there is now a requirements file to run pip install -r requirements.txt and install those dependencies as well.