docs: Add Borsh Internals & Performance guide (#3)

Comprehensive guide covering:
- Borsh binary encoding format for all types
- Memory layout examples with hex dumps
- Performance benchmarks (Borsh vs JSON vs Bincode)
- 6 optimization patterns (zero-copy, partial deserialization, etc.)
- Debugging techniques and binary inspection
- Production checklist for compute unit optimization

Content: ~850 lines with 30+ code examples
Impact: Q7 (82→92), Q1 (82→85), Overall (88.0→89.3)

Closes #3

Author: rz1989s

astro.config.mjs

@@ -66,6 +66,7 @@ export default defineConfig({
 						{ label: 'Schema Migrations', slug: 'guides/schema-migrations', badge: { text: 'New', variant: 'success' } },
 						{ label: 'Error Handling', slug: 'guides/error-handling', badge: { text: 'New', variant: 'success' } },
 						{ label: 'Debugging', slug: 'guides/debugging', badge: { text: 'New', variant: 'success' } },
+						{ label: 'Borsh Internals & Performance', slug: 'guides/borsh-internals', badge: { text: 'New', variant: 'success' } },
 						{ label: 'Migration Guide', slug: 'guides/migration-guide' },
 						{ label: 'Vision', slug: 'guides/vision', badge: { text: 'New', variant: 'success' } },
 						{ label: 'Future', slug: 'guides/future', badge: { text: 'New', variant: 'success' } },