Add downloadable version of the docs as PDF

[retiutut]

This can likely be done during CI/CD using a tool.

Depending on the size of the PDF, maybe the best option is to include this in the gh-pages branch and use a relative link on a page to link to it. This way, the link to this file would remain the same, though we need to overwrite this file on every deploy so that it remains current.

[retiutut]

Just found this today 🔥 https://github.com/signcl/docusaurus-prince-pdf