Add cross-references to each wgpu type's documentation.

Added:
* WebGPU specification links.
* How to create an instance, if it is not all-public-fields.
* What function or enclosing type this type is used for.

(The latter two are not present for `wgpu-types` items due to the
dependency running the wrong direction for that — I saw existing
`../wgpu/` links, but those do not work on https://docs.rs/wgpu/.)

This commit specifically does not:
* substantially revise existing language, except where necessary to fit
  the new information, or when there was an obvious trivial improvement
  like adding a link to support "how to use this".
* add similar information to the documentation of types not directly
  corresponding to the WebGPU specification