Docs: Directly reference upstream spec for each item in webgpu.h

[kainino0x]

Either with a link to the JS spec, or by embedding documentation from the spec into the generated header.

Originally posted by @kainino0x in #25 (comment):

Apr 11 meeting:

• doc jam - how should we do this?
• KN: add links to spec in yaml?
  • pull text from it at codegen time? (but the names will differ)
  • or just generate links to the spec?
• Generator pulling from spec is worth exploring - from html
• Explore having the spec repo itself generate some parseable (e.g. json) documentation out of the spec, similar to what types generator does today — then use this for both typescript and C