docs: restructure README into docs/ pages

Move detailed feature documentation from README.md into organized docs/ files:
- docs/server.md: tools, resources, prompts, logging, completions, sampling,
  elicitation, transports (server-side)
- docs/client.md: connecting to servers, transports (client-side), using
  tools/resources/prompts/completions
- docs/protocol.md: progress notifications, pagination

README.md slimmed to entry point with project description, install, quickstart,
and links to docs/. No new content — pure reorganization of existing documentation.

README: 2,562 lines -> 187 lines.

Author: felixweinberger