Use Nunito font as default sans-serif font

* `style.css` includes a CSS snippet adapted from the Stack Overflow answer https://stackoverflow.com/a/52226523/ by [binaryfunt][binaryfunt] which is licensed under [**CC BY-SA 4.0**][cc-by-sa-4.0]. More details can be found in the respective file.

* The [icon this repository uses](https://gitlab.cl.uni-heidelberg.de/uploads/-/system/project/avatar/720/web-design-padded.png) was made by [monkik][monkik] from https://www.flaticon.com and is licensed under the **Flaticon License**.

* `js/lib/qrcode/*` contains a JavaScript library to create QR codes, which is licensed under The MIT License (MIT) and authored by davidshimjs (https://davidshimjs.github.io/qrcodejs/)

* `fonts/lib/Nunito/*` is the Font Nunito, licensed under SIL OFL.

[original]:https://gitlab.cl.uni-heidelberg.de/fachschaft/fscoli-next/-/tags/original

[personads]:https://personads.me/

Copyright 2014 The Nunito Project Authors (https://github.com/googlefonts/nunito)

This Font Software is licensed under the SIL Open Font License, Version 1.1.

This license is copied below, and is also available with a FAQ at:

https://openfontlicense.org

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SIL OPEN FONT LICENSE Version 1.1 - 26 February 2007

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@font-face {

    font-family: "Nunito";

    font-weight: 200 1000; /* We support weights from 200 up to 1000 */

    font-style: normal;

    src:

        local("Nunito"), 

        url("./Nunito-VariableFont_wght.ttf");

}

@font-face {

    font-family: "Nunito";

    font-weight: 200 1000; /* We support weights from 200 up to 1000 */

    font-style: italic;

    src:

        local("Nunito"), 

        url("./Nunito-Italic-VariableFont_wght.ttf");

}