v2.2.1 — docs sweep for v2.1/v2.2 services

Docs-only patch on top of v2.2.0. No code, no config schema, no behaviour
changes — only documentation accuracy.

README
- Intro, resource management, usage curl examples, Logs and Debugging
  table, troubleshooting, and the "Full usage guide" blurb all now thread
  predictalot / mailbox / telethon mentions where the surrounding section
  was listing capabilities.

docs/usage.md
- Four new sections: Web search (SearXNG MCP), Time-series forecasting
  (predictalot), Email gateway (mailbox), Telegram client (telethon).

docs/testing.md
- Test-suite list extended with the new predictalot, mailbox, telethon
  suites + the opt-in mailbox e2e gate.

litellm/config/mcp/mcp.yaml
- mcp_tools aggregator description now lists search_web alongside
  generate_image and generate_tts.