Add models to the spec appendix

[anssiko]

It might be a good idea to move the first-wave models doc, currently a separate markdown file in this repo, into a spec appendix section. That way we'd have all the data we've used for mapping the identified use cases to models to ops in the same place. The spec already hosts the use cases and the definition of the ops.

Edit: Wave 2 and 3 models are documented in WebNN Operator Update
Wave 3 and discussed in #375. It'd be good to have all the Waves documented in the same place, not scattered around.

cc @dontcallmedom