Clarify package.json vs deno.json for Deno Desktop frameworks

[Rohan170603]

Problem
Framework projects scaffolded with deno init --npm generate a package.json, but the desktop manifest key that Deno Desktop expects does not belong there. Developers are unsure whether to keep both files, fold one into the other, or where to put desktop configuration.

---

Solution

• Added a dedicated section explaining that package.json declares npm dependencies and deno.json configures Deno-specific settings, including the desktop manifest.
• Explicitly stated that Deno Desktop ignores the desktop key in package.json.
• Provided a concrete example showing both files with correct contents.
• Added a short FAQ answering "Do I need both files?"

---

Changes

• Modified runtime/desktop/frameworks.md to include the new section, example, and FAQ.

---

Checklist

• Clarified the distinct roles of package.json and deno.json.
• Added warning that package.json desktop keys are ignored.
• Included example snippets for both files.
• Added FAQ entry.
• Verified Markdown formatting.
• Updated last_modified date.

Closes #3379.