docs: rewrite README to reflect spec-compliant API, callbacks, and new param shapes

- Update all JSON-RPC method docs to current request/response formats:
  sendTransaction flat params, sendTransactionMultichain array,
  getStatus single id+logs, getCapabilities chain-keyed map,
  getFeeData rate as tokens-per-native
- Document webhook callback feature (context.callbackUrl)
- Document task ID semantics (auto-generated or client-provided)
- Document all spec error codes (4200-4214)
- Update Docker section to reference 0xpartha/relayx:latest
- Update test count (28), storage key reference, project structure
- Remove stale old-format curl examples and outdated compliance tables

Author: ch4r10t33r