Draft: Jinja templating

[This MR is only meant as a demo, conflicts with other MRs are therefore irrelevant, conda base branch to have a newer modules.yaml file]

I would like to switch from our custom templating (##VARIABLE##) systax to jinja. This MR outlines a possible new structure and summarises the new templating in a new documentation file.

The current templating is pretty simple, so we do not (yet) gain much from switching to jinja but I expect that to change once more logic is moved to the software manager (e.g. auto-detection of system packages).

@henning.glawe, @jmello and @hans.fangohr Please review the new structure, read (best in preview mode) the new documentation, and comment/request changes. Once we agree on a new structure I will update the software-manager accordingly. As we want to release soon I'd ask you to do this with high priority.