MVP design document

This branch contains a design document for the implemetation of the MVP.

The MR is marked as draft because I would like to first merge the PerceiverIO branch into main and finish the corresponding part of the design doc.

There will be empty sections (physical models, for instance) that will be completed later starting new branches from main.

I have already some interrogations with the choice of org-mode for this doc. Even just as a markup language I prefer it to markdown or RST, and even VSCode supports org-mode editing. But some people may find that we should use other formats. We can always convert with Pandoc before merging.

I would have gone further and use full org-mode capabilities (references, cross-references, babel, etc.). With a pixi dependency to Emacs, we can automatically generate an export in any format, which can be useful for publications, etc. And we could use TikZ for generating SVG figures, etc. But for now, we can stick to this README-like doc.