chore: remove Ask Fern Discord integration from docs

[dannysheridan]

Summary

Removes all Ask Fern Discord bot documentation and API references as part of the broader Discord integration removal (companion to fern-api/fern-platform#9019).

Deleted:

• discord-bot.mdx page and 5 associated screenshot assets
• Discord endpoints, schemas, and discord_enabled field from the FAI OpenAPI spec
• Discord from the navigation sidebar and hidden Scribe API layout

Updated:

• AI features overview, Slack app, and Ask Fern overview pages — removed Discord mentions/links
• endpoint-schema-snippet.mdx — replaced POST /discord/install example with POST /chat/{domain}
• docs.yml — added redirect from old Discord bot URL → Ask Fern overview

Intentionally kept:

• Footer link discord field in docs-yml/definition/docs.yml and site-level-settings.mdx (general docs.yml feature, not Ask Fern)
• Discord mention in metadata.mdx (social sharing context)
• Historical changelog entry in 2025-10-27.mdx

Review & Testing Checklist for Human

• Verify the EndpointSchemaSnippet example change: the "Request query" section now uses POST /chat/{domain} — confirm this endpoint has query parameters so the example renders correctly (previously used the now-deleted POST /discord/install)
• Check the redirect works: /learn/docs/ai-features/ask-fern/discord-bot → /learn/docs/ai-features/ask-fern/overview
• Confirm no broken links remain on the Ask Fern overview, Slack app, and AI features pages after Discord card/link removal

Suggested test plan: Deploy the preview and navigate to /learn/docs/ai-features/ask-fern/overview, /learn/docs/ai-features/ask-fern/slack-app, /learn/docs/ai-features/overview, and /learn/docs/writing-content/components/endpoint-schema-snippet to verify pages render cleanly without dead links or empty components.

Notes

• This PR must land alongside or after fern-api/fern-platform#9019 which removes the Discord backend, SDK types, and dashboard references.
• The FAI OpenAPI spec in this repo is a local copy; if it's ever regenerated from the backend, the Discord removal in fern-platform ensures it won't reappear.

Link to Devin session: https://app.devin.ai/sessions/922f15fac6e041618b864233d012499e
Requested by: @dannysheridan

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[github-actions]

🌿 Preview your docs: https://fern-preview-devin-1774551148-remove-discord-integration.docs.buildwithfern.com/learn

Here are the markdown pages you've updated:

• learn/docs/ai-features/overview
• learn/docs/ai-features/ask-fern/slack-app
• learn/docs/ai-features/ask-fern/overview
• learn/docs/writing-content/components/endpoint-schema-snippet