Expand converter documentation with registration methods and priority

[fdcastel]

Motivation

The existing converter documentation covers the basics but doesn't explain:

• The three registration methods and their priority
• Converter factories for open generic / type-family scenarios
• The CurrentKey context available to converters
• A clear precedence summary

Changes

Rewrites site/docs/serialization/converters.md to cover:

1. Registration methods — member-level attribute, options-level, type-level attribute with code examples
2. Priority order — explicit ordering from highest (member attribute) to lowest (object converter)
3. Converter factories — YamlConverterFactory pattern with CanConvert/CreateConverter example
4. CurrentKey context — how custom converters can access YamlReader.CurrentKey for context-dependent logic
5. Precedence summary table — complete resolution chain including IParsable<T> fallback