add highlighting for shell input/output

[sbruder]

This implements highlighting for shell input/output. To use it, two adjacent code blocks must have shell-session and none as their highlighting language.

As for how to highlight it, I copied my approach from the Summer of Nix homepage (which itself is based on the German Ubuntu Wiki) and inverted the text/background colours. I’m not sure if that is the best way to handle this here, because the output also gets highlighted and might look weird with inverted colours.

Also, I noticed that Sphinx does not check if the files in html_static_path have changed on a rebuild, so they will not be replaced unless a full build is triggered.

cc @fricklerhandwerk

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