docs: Add comprehensive API reference documentation (Issue #16)

Created 3 complete API reference pages:

1. CLI Commands (api/cli-commands.md)
   - All 4 commands: generate, validate, init, check
   - Usage examples, options, exit codes
   - Common errors and troubleshooting
   - CI/CD integration examples

2. Type System (api/types.md)
   - Complete type mapping table (Rust ↔ TypeScript)
   - Primitives: u8-u128, i8-i128, bool, String
   - Solana types: PublicKey, Signature
   - Complex types: Vec, Option
   - Enums: Unit, Tuple, Struct variants
   - Best practices and constraints

3. Attributes (api/attributes.md)
   - #[solana] - Solana-specific types
   - #[account] - Anchor account macro
   - Context-aware generation
   - Valid/invalid combinations
   - Troubleshooting common errors

Updated sidebar navigation to include API Reference section.

Progress: 75% complete on Issue #16

Author: rz1989s

astro.config.mjs

@@ -48,6 +48,14 @@ export default defineConfig({
 						{ label: 'Quick Start', slug: 'getting-started/quick-start' },
 					],
 				},
+				{
+					label: 'API Reference',
+					items: [
+						{ label: 'CLI Commands', slug: 'api/cli-commands' },
+						{ label: 'Type System', slug: 'api/types' },
+						{ label: 'Attributes', slug: 'api/attributes' },
+					],
+				},
 				// TODO: Add these sections as we create the content
 				// {
 				// 	label: 'Guides',
@@ -59,14 +67,6 @@ export default defineConfig({
 				// 	],
 				// },
 				// {
-				// 	label: 'API Reference',
-				// 	items: [
-				// 		{ label: 'CLI Commands', slug: 'api/cli-commands' },
-				// 		{ label: 'Parser', slug: 'api/parser' },
-				// 		{ label: 'Generators', slug: 'api/generators' },
-				// 	],
-				// },
-				// {
 				// 	label: 'Examples',
 				// 	items: [
 				// 		{ label: 'Gaming Platform', slug: 'examples/gaming' },