Improve Marketplace presentation and discoverability

[Perdolique]

Summary

Improve the Marketplace presentation, repository metadata, and discoverability of Zero Reference.

Scope

• Add a distinctive 256x256 PNG extension icon and include it in the VSIX.
• Replace the current low-resolution GIF with a current, high-DPI demonstration of conservative detection and result removal after a reference is added.
• Expand the README with:
  • the zero-only product promise;
  • comparison with VS Code's built-in reference CodeLens;
  • supported declarations and languages;
  • configuration and commands;
  • external/dynamic reference limitations;
  • exclusion and suppression examples when available.
• Update repository and issue URLs to the canonical Perdolique/vscode-zero-reference location.
• Evaluate Linters and Programming Languages Marketplace categories instead of relying only on Other.
• Keep contributor/release instructions separate from the user-focused opening sections.

Acceptance criteria

• vsce package completes without warnings about icon, links, or images.
• The packaged VSIX contains the icon, README assets, and only required runtime files.
• README links and images render correctly from both GitHub and Marketplace base URLs.
• The first viewport explains what the extension does, what it does not prove, and how it differs from built-in reference counts.
• The Marketplace listing displays the new icon, categories, and current repository links after publication.