Clarify pacakge.json vs deno.json for deno desktop + frameworks

The skeletons of the frameworks at https://docs.deno.com/runtime/desktop/frameworks/ are generated with a `package.json` (e.g. by `deno init --npm vite`). The schema of that `package.json` doesn't have the `desktop` key that `deno desktop` expects. It would help to have some guidance on the best practice for this situation:
- keep both `package.json` and `deno.json`?
- fold `package.json` into `deno.json`?

Start with the desktop frameworks page at docs.deno.com/runtime/desktop/frameworks/ and inspect the skeletons generated by `deno init --npm vite`. Check how `deno desktop` reads `package.json` and `deno.json`, then document the recommended relationship between them. Done means the page clearly explains whether to keep both files or consolidate configuration, with guidance that applies to the listed framework skeletons.

Written by the indexing model from the issue text.