[Major] Transfer docs to ardusub-zola

#227 / #229 require relatively major restructures of the current repo. Since [GitBook is no longer maintained](https://snyk.io/advisor/npm-package/gitbook) it's also well-advised to transfer to a new site-building framework.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](https://www.getzola.org/) 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](https://github.com/bluerobotics/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).

Start by reviewing the restructuring requirements in issues #227 and #229, then inspect the current GitBook content and the ardusub-zola repository. Done means all relevant documentation, including unresolved but still relevant issues, has been transferred to ardusub-zola and this repository no longer needs updates.

Written by the indexing model from the issue text.