DOCS: Create AboutCode doc system

To improve and standardize the documentation across AboutCode projects, we plan to implement a documentation system on https://aboutcode.readthedocs.io/. This system will consist of pages inspired by the Divio documentation system, which explains the [Diátaxis framework](https://diataxis.fr/) and its application to AboutCode projects.

Beyond outlining how project docs should be structured it the docs system should also provide guidance on README, CONTRIBUTING and repository descriptions.