fix: link to docs in JSON import error message (#34611)

Adds a `docs:` hint pointing users at
https://docs.deno.com/examples/importing_json/ when they import a JSON
module without an `import attribute`. The link is shown for both
`UnsupportedMediaType` (running `import data from "./data.json"` with no
attribute) and `InvalidTypeAssertion` cases.

Closes denoland/deno#26016

Co-authored-by: divybot <divybot@users.noreply.github.com>
Co-authored-by: Divy Srivastava <me@littledivy.com>