Add third-party tools integration docs and embed snippet example

[devin-ai-integration]

Summary

Expands the "Connect other integrations" section on the integrations overview page (fern/products/docs/pages/integrations/overview.mdx) to document the third-party tool integration capabilities of custom JavaScript. Previously, this section was a single sentence linking to the custom JS docs. Now it includes:

• A renamed heading: "Connect other integrations via custom JavaScript"
• A description clarifying that custom JS is for tools not natively supported in docs.yml
• A categorized list of example tools that require custom JS: analytics (Amplitude, Heap, Plausible), session recording (Hotjar, LogRocket, Microsoft Clarity), support/chat (Zendesk, Crisp, Drift), and tag managers (Adobe Launch, Tealium)
• A Hotjar embed snippet as a concrete code example, with a docs.yml reference
• A closing link back to the custom JS setup page

Also updates the "Common use cases" list on the custom CSS & JS page (custom-css-js.mdx) to flesh out the cross-reference to the integrations page, with bolded labels and a note that custom JS is for tools not natively supported in docs.yml.

No new pages, components, or features are introduced — this is a content-only update across two existing pages.

Updates since last revision

• Removed tools that already have native docs.yml analytics support (Google Analytics, Google Tag Manager, Segment, PostHog, FullStory, Intercom, Mixpanel) from the custom JS recommendation lists.
• Replaced the Google Tag Manager code example with a Hotjar embed snippet, since GTM has native docs.yml support.
• Updated wording on both pages to clarify that custom JS is specifically for tools Fern doesn't natively support.

Review & Testing Checklist for Human

• Anchor link change: The heading slug changed from #connect-other-integrations to #connect-other-integrations-via-custom-javascript. Verify no internal pages or external sources link to the old anchor. If so, consider adding a redirect or reverting to the original heading text.
• Tool list accuracy: Confirm none of the listed tools (Amplitude, Heap, Plausible, Hotjar, LogRocket, Microsoft Clarity, Zendesk, Crisp, Drift, Adobe Launch, Tealium) have native docs.yml support that was missed. Conversely, verify no tools that do require custom JS were left out if they're commonly requested.
• Cross-reference link: The custom-css-js.mdx page now links to /docs/integrations/overview#connect-other-integrations-via-custom-javascript. Confirm this anchor resolves correctly in the preview.
• Code example rendering: Verify the Hotjar CodeBlock on the integrations page renders correctly (syntax highlighting, no broken JSX). The snippet uses YOUR_HOTJAR_ID as a placeholder — confirm this is acceptable as a documentation example.
• Preview rendering: Check both updated pages in the preview deployment to ensure bulleted lists, bold labels, inline code, and CodeBlocks render correctly.

Test plan: Open the preview links for both pages, verify the content displays as expected, and click the cross-reference links in both directions to confirm they resolve.

Notes

• Vale checks pass with 0 errors and 0 warnings. Two pre-existing suggestions remain (Custom JavaScript heading casing and first-person We — both pre-existing).
• All inline links use /docs/customization/custom-css-js#custom-javascript, matching the existing link convention.
• The <Markdown src="/snippets/enterprise-plan.mdx"/> snippet is preserved from the original section.

Link to Devin session: https://app.devin.ai/sessions/03964d70af314ed0bbd0555d1c9cb229

[devin-ai-integration]

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[fern-api]

Requested by: jon
Slack thread: View conversation

[github-actions]

🌿 Preview your docs: https://fern-preview-fern-add-third-party-integration-docs.docs.buildwithfern.com/learn

Here are the markdown pages you've updated:

• learn/docs/customization/custom-css-js
• learn/docs/integrations/overview

[github-actions]

📝 [vale] _{reported by reviewdog 🐶}
[FernStyles.Acronyms] 'GTM' has no definition.