docs: flatten directory structure

Move documentation from nested guides/ to flat docs/ structure:
- guides/api-reference.md → api-reference.md
- guides/builder-pattern.md → builder-pattern.md
- guides/usage-examples.md → usage-examples.md

Author: jdalton

docs/guides/api-reference.md docs/api-reference.mddocs/guides/api-reference.md renamed to docs/api-reference.md

@@ -468,7 +468,7 @@ Additional factory methods available:
 - `PackageURLBuilder.deb()` - Debian packages
 - `PackageURLBuilder.rpm()` - RPM packages
 
-See [Builders Guide](./BUILDERS.md) for complete list.
+See [Builder Pattern Guide](./builder-pattern.md) for complete list.
 
 ---
 
@@ -910,6 +910,6 @@ if (ResultUtils.isErr(result)) {
 
 ## See Also
 
-- [Examples](./EXAMPLES.md) - Practical usage examples
-- [Builder Guide](./BUILDERS.md) - Comprehensive builder patterns
+- [Usage Examples](./usage-examples.md) - Practical usage examples
+- [Builder Pattern Guide](./builder-pattern.md) - Comprehensive builder patterns
 - [Package URL Spec](https://github.com/package-url/purl-spec) - Official specification

docs/guides/builder-pattern.md docs/builder-pattern.mddocs/guides/builder-pattern.md renamed to docs/builder-pattern.md

@@ -558,6 +558,6 @@ Some package types apply automatic normalization:
 
 ## See Also
 
-- [API Reference](./API.md) - Complete API documentation
-- [Examples](./EXAMPLES.md) - Practical usage examples
+- [API Reference](./api-reference.md) - Complete API documentation
+- [Usage Examples](./usage-examples.md) - Practical usage examples
 - [PURL Types Specification](https://github.com/package-url/purl-spec/blob/master/PURL-TYPES.rst) - Official type definitions

docs/guides/usage-examples.md docs/usage-examples.mddocs/guides/usage-examples.md renamed to docs/usage-examples.md

@@ -613,6 +613,6 @@ if (isNpmPackage(purl) && hasVersion(purl)) {
 
 ## See Also
 
-- [API Reference](./API.md) - Complete API documentation
-- [Builder Guide](./BUILDERS.md) - Builder pattern reference
+- [API Reference](./api-reference.md) - Complete API documentation
+- [Builder Pattern Guide](./builder-pattern.md) - Builder pattern reference
 - [Package URL Spec](https://github.com/package-url/purl-spec) - Official specification