Add “Advanced Feature” callout to relevant docs pages

[SarahSoutoul]

🔎 Previews:

• https://clerk.com/docs/pr/ss-docs-11138/guides/dashboard/dns-domains/satellite-domains
• https://clerk.com/docs/pr/ss-docs-11138/guides/dashboard/dns-domains/proxy-fapi
• https://clerk.com/docs/pr/ss-docs-11138/guides/development/custom-flows/overview

What does this solve?

Linear: https://linear.app/clerk/issue/DOCS-11138/create-callout-for-advanced-features-in-terms-of-engagement

This change reintroduces guidance that previously existed in the "Advanced" section of the docs, which was removed during the new IA rollout. Some workflows and configurations sit outside typical usage patterns and require clear expectations for customers engaging with Support. This callout helps set those expectations and aligns with the Terms of Engagement (specifically Point 3: Limits to Our Service). It also provides a consistent way to flag these advanced scenarios across the docs.

What changed?

• Added a reusable "Advanced Feature" callout partial that can be injected in any pages in the future.
• Updated relevant pages to include the new callout. As @royanger mentioned in this Slack thread, the list starts with all custom flows, proxying the frontend api and satellite domains.
• Important note: for custom flows, I did things a bit differently. Instead of using the partial, I incorporated the info directly in the Custom flows overview. If we prefer to have the callout in every single custom flow page, I can change that, but thought it worked better like this.

Checklist

• I have clicked on "Files changed" and performed a thorough self-review
• All existing checks pass

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Project	Deployment	Preview	Updated (UTC)
clerk-docs	Ready	Preview	Dec 2, 2025 9:26pm

[alexisintech]

this looks good to me! i say ship