keep toggle bar persistent

[matteodelucchi]

Currently the toggle bar is always shown. When it's turned-off on one page, it comes back on the next.
When on slide mode, keep the state of the toggle-bar as in previous slide. E.g. by default either on or off, but when once turned off on one slide, keep this setting for the proceeding slides.

[matteodelucchi]

• check sphinx book theme if it's an option (evtl. conf.py)
• check if this is possible in css
• otherwise, change it in javascript.

see https://github.com/t4d-gmbh/using-git-in-academia/blob/main/source/_static/slides.css

[matteodelucchi]

full-screen mode is also affected...

[matteodelucchi]

That's too tedious for now and is an issue that belongs to the sphinx-book-template. The same behaviour can be observed on their doc page.
I think an approach similar to that used to keep the dark/light mode might be sensible (see here) to address it.