docs(api-def): document x-fern-explorer for AsyncAPI channels

[devin-ai-integration]

Summary

x-fern-explorer — the extension that controls whether the interactive API Explorer is offered for an API — now works in AsyncAPI specs, not just OpenAPI. WebSocket API owners can keep the Playground off for channels where opening a live connection is undesirable, set that default once at the document root, and re-enable it per channel. This mirrors the control they already have over HTTP endpoints, and channels registered before the change keep showing the Playground.

Implements docs for: feat(cli): add x-fern-explorer support for AsyncAPI specs (fern-api/fern#17233) and feat(docs): add includeInApiExplorer for WebSocket channels + honor it in the playground (fern-api/fern-platform#13405).

Pages changed

• New fern/products/api-def/asyncapi/extensions/explorer.mdx ("API Explorer control (AsyncAPI)") — mirrors the shape of its sibling AsyncAPI extension pages: root-level x-fern-explorer: false, per-channel x-fern-explorer: true override, the resolution rule (channel value, else document value, else enabled), the effect (Playground button hidden for the channel), and that non-boolean values error at import. Tabs cover AsyncAPI 3.0 and 2.x, whose channel objects are spelled differently.
• asyncapi/extensions/overview.md — new row in the available-extensions table.
• api-def.yml — registers the page under AsyncAPI > Extensions.
• openapi/extensions/explorer.mdx and docs/pages/api-references/api-explorer.mdx — one-sentence pointers to the new page, so readers arriving from the OpenAPI extension or the Explorer feature page find the WebSocket behavior.
• New Docs changelog entry 2026-08-19.mdx.

Checks: pre-commit run --files <changed> clean (the one Vale error is pre-existing on an untouched line of api-explorer.mdx), fern check --warnings reports 0 errors.

Link to Devin session: https://app.devin.ai/sessions/9ca03cb676de48119e35825b911bdb84

[devin-ai-integration]

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[github-actions]

🌿 Preview your docs: https://fern-preview-devin-1787159329-asyncapi-explorer-extension.docs.buildwithfern.com/learn

Here are the markdown pages you've updated:

• learn/api-definitions/asyncapi/extensions/api-explorer-control
• learn/api-definitions/openapi/extensions/api-explorer-control
• learn/docs/api-references/api-explorer
• learn/docs/changelog/2026/8/19