[Major] Transfer docs to ardusub-zola

[ES-Alexander]

#227 / #229 require relatively major restructures of the current repo. Since GitBook is no longer maintained it's also well-advised to transfer to a new site-building framework. We may as well do both major changes at once and save ourselves some pain.

After internal discussion and looking at a few different options, it seems that Zola is a solid GitBook alternative, which should make our docs easier to use and maintain while also being an actively developed framework (so less prone to known security issues and the like).

Accordingly, the ardusub-zola repo is planned as the new home for ardusub.com docs. This repo will stop receiving updates once the relevant content has been transferred (including any unresolved and still relevant issues).