Add draft of the specification of validation

[pl-semiotics]

This PR adds a spec/document folder with a draft of the beginnings of a spec document, currently providing a formalisation of validation rules for the component model.

[lukewagner]

Woohoo, excited to see this; great work! Just to lay out my plan, I'll be incrementally reviewing the PR in detail over the coming weeks, submitting comments piecemeal.

[pl-semiotics]

@lukewagner Updated with the current structure of resource types.

I also removed the checkin of docs/, since it doesn't seem to have worked to provide an easy rendered version & it causes a lot of noise in git. See the rendered version for now; if this is merged, I'll set up CI hooks + GH Pages to get a CI rendered version.

[pl-semiotics]

@lukewagner As discussed, this version has been de-Sphinx'd. It's mostly in line with the reference implementation now. The changes from the previous version are mostly minor (but important) fixes; the largest presentational change is also the exception to that (i.e. is not yet done in the reference implementation), but should not actually change almost anything: existential quantifiers have been moved from instancetype_e to componenttype_e and deftype_e, since they are (and must be) always hoisted to one of them anyway.

[lukewagner]

Sorry for the slow reply here, but excited to see this update! Could you perhaps also include a rendered .pdf in the document/ directory? Maybe we remove it before things are final, but at least in the interim, it would be convenient.