Add comprehensive YARD documentation

[nomadium]

This commit adds extensive YARD documentation to improve API discoverability and developer experience. Documentation coverage increased from ~76% to ~94%.

Key additions:

• Module and class-level documentation with usage examples
• @param, @return, @raise, and @option tags for all public methods
• @example blocks showing common usage patterns
• @see references to related classes and RFC sections
• @api private markers for internal implementation details

Documented modules:

• Linzer (main module with sign/verify/signature_base)
• Key classes (Ed25519, ECDSA, HMAC, RSA, RSAPSS, JWS)
• Key::Helper (key generation and loading methods)
• Signer and Verifier modules
• Signature class
• Message class and adapters (Rack, Net::HTTP, http.rb, Generic)
• HTTP client module and SignatureFeature for http.rb
• Rack::Auth::Signature middleware
• Common utilities and Options

The documentation follows YARD conventions and includes:

• RFC 9421 section references where applicable
• Security considerations and recommendations
• Algorithm-specific notes (e.g., RSA-PSS non-determinism)
• Cross-references between related classes