Using `sphinxext-photofinish` for creating responsive images

Hi.

At https://github.com/crate/cloud-docs/pull/34, we discussed image sizes. Maybe we want to support both large and small screens at the same time optimally, not needing to rely on fast pipes everywhere, i.e. saving bandwidth.

On this matter, we may want to make use of [sphinxext-photofinish](https://github.com/wpilibsuite/sphinxext-photofinish):

> Photofinish is a sphinx extension for creating [responsive](https://developer.mozilla.org/en-US/docs/Learn/HTML/Multimedia_and_embedding/Responsive_images) images to your Sphinx site. This has several benefits:
>
> - Lower bandwidth cost for browsers and devices that don't need the higher resolution version
> - Higher resolution images for higher resolution devices
>
> Photofinish can dramatically increase the loading time of your Sphinx website.

With kind regards,
Andreas.

/cc @matkuliak, @msbt