Add documentation with examples for all previously undocumented features:

Server (docs/server.md - 260 lines added):
- Tools: audio results, change notifications
- Resources: binary reading, subscribing, unsubscribing
- Prompts: embedded resources, image content, change notifications
- Logging: setting level
- Elicitation: enum values, complete notification

Client (docs/client.md - 109 lines added):
- Roots: listing, change notifications
- SSE transport (legacy client)
- Ping, logging

Protocol (docs/protocol.md - 110 lines added):
- Ping, cancellation, capability negotiation
- Protocol version negotiation, JSON Schema 2020-12

All code snippets verified against SDK source.