[document] First stab at specifying validation

[rossberg]

Please see test rendering on https://webassembly.github.io/spec/

[sunfishcode]

When we discussed this, we agreed that formal notation would be placed in an appendix, rather than being used within the main document itself.

[lukewagner]

Having spent some time scanning over this on a few occasions now, I think I really like this current interleaved formatting, for a few reasons:

• it seems easier to keep the two precisely coherent and spot discrepancies when they are juxtaposed
• as a future frequent user of this document, I'll definitely benefit from having the formal rules right next to bulleted explanation
• with the mini-tutorial given in conventions.html#formal-notation, I expect anyone not already familiar with the notation could pick it up by first observing the basic style and then learning by example from reading a few rules
• there's a pretty minimal impact on readability even if you just want to skip over it
• the interleaved heading / summary / notation / explanation form matches the "Structure" section which I also really like as is; in general, I think this is a good high-level style to use throughout the doc

IIRC, this also matches @dherman's recommendation last time we discussed this organizational question. Happy to have any comments from you too Dave.

[lukewagner]

Overall, lgtm. I'm expecting we'll iterate more as everything comes together.