✨ NEW: Reinstate appendices from 2020 tutorial as modules

[mbercx]

The 2020 tutorial had several appendices:

https://aiida-tutorials.readthedocs.io/en/latest/pages/2020_Intro_Week/index.html#appendices

Which are currently missing from the new restructured content. Since they can contain interesting content, and e.g. the Querying tutorial referred to some of them, we should look into reinstating these appendices as modules, or as part of other modules.