Add pro plan callout to subpath section in custom domain docs

[devin-ai-integration]

Add pro plan callout to subpath section in custom domain docs

Summary

Adds the shared pro-plan callout (/snippets/pro-plan.mdx) to the Subpath accordion in the Setting up your domain page. This makes it clear that subpath custom domains are a Pro/Enterprise feature, consistent with how the callout is used elsewhere in the docs.

Also removes minor trailing whitespace on the last line of the file.

Review & Testing Checklist for Human

• Verify the callout renders correctly inside the Subpath accordion on the preview deployment — it should appear as a warning banner before the instructional text
• Confirm the placement feels right (above the description paragraph, before the Steps)

Notes

• The <Markdown src="/snippets/pro-plan.mdx"/> pattern is already used across many pages (e.g., audiences, pagination, retries, idempotency docs)
• Link to Devin run
• Requested by: Fern Support

[devin-ai-integration]

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[github-actions]

🌿 Preview your docs: https://fern-preview-d5b92d65-4e0d-4733-8881-8ef21a40fccd.docs.buildwithfern.com/learn

Here are the markdown pages you've updated:

• learn/docs/preview-publish/setting-up-your-domain