Document WebSocket support

[Kludex]

Documents the native WebSocket support added in #1042, following the same structure as the SSE docs (#1050):

• New docs/websockets.md guide covering opening sessions, sending/receiving messages, subprotocols, keepalive pings, configuration, error handling, and testing ASGI applications with ASGIWebSocketTransport.
• WebSockets entry in the nav, after Server-Sent Events.
• API reference: top-level websocket() helper, websocket member on Client/AsyncClient, and WebSocketSession/AsyncWebSocketSession sections.
• wsproto listed in the optional dependencies on the index page.

Docs build passes with no new warnings.

AI Disclaimer

This PR was developed with the assistance of either Claude or Codex. I've reviewed and verified the changes.

[github-actions]

Docs preview:

[codspeed-hq]

Merging this PR will not alter performance

✅ 15 untouched benchmarks
⏩ 7 skipped benchmarks¹

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_{Comparing document-websockets (ac1e562) with main (2cb02fe)}

Footnotes

1. 7 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[chatgpt-codex-connector]

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[cubic-dev-ai]

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