Document blocked headers for WebSocket Playground proxy

[devin-ai-integration]

Summary

Adds a "Blocked headers" subsection under the WebSocket Playground section of the API Explorer docs page. This documents the set of headers that the WebSocket proxy strips during the connection handshake for security reasons, so users know which headers won't be forwarded to upstream servers.

This accompanies the security fix in fern-platform#8267, which adds header sanitization to the WebSocket proxy worker.

Review & Testing Checklist for Human

• Verify the blocked header list matches the code: Cross-reference the list in this PR against the DENIED_HEADERS set in packages/workers/proxy/src/websocket.ts in fern-platform#8267. If either PR's list changes, they must stay in sync.
• Confirm placement: The new ### Blocked headers subsection is placed under ## WebSocket Playground. Verify this is the best location — an alternative could be the WebSocket definition page or the WebSocket reference page.
• Check Vale/style compliance: Vale wasn't run locally. Confirm the new content passes Vale linting in CI, especially sentence-case heading rules and the conciseness/verbosity guidelines.

Notes

• The companion code PR is fern-platform#8267 — it adds the denylist, CRLF injection checks, logging, and unit tests.
• Link to Devin Session: https://app.devin.ai/sessions/4ad34a355a7a4d76b596787e7ef7f5c2
• Requested by: @davidkonigsberg

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