docs: add in-depth comparison vs Bun and Deno (#249)

Adds docs-site/guide/vs-bun-deno.md covering native addon support,
bundle format, cross-compile, bytecode, binary size, plus a real
@anthropic-ai/claude-code bundling case study across all three tools.
Includes a self-critical verdict section to prevent the page from
reading as marketing.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: robertsLando

docs-site/.vitepress/config.ts

@@ -77,6 +77,7 @@ export default withMermaid({
             { text: 'What is pkg?', link: '/guide/' },
             { text: 'Getting started', link: '/guide/getting-started' },
             { text: 'SEA vs Standard', link: '/guide/sea-vs-standard' },
+            { text: 'pkg vs Bun vs Deno', link: '/guide/vs-bun-deno' },
             { text: 'Migration from vercel/pkg', link: '/guide/migration' },
           ],
         },