docs: add coding-agent onboarding page at /learn/home/get-started

[matlegault]

Summary

Adds a hidden page under the Home product, fern/products/home/pages/agent-onboarding.mdx, published at https://buildwithfern.com/learn/home/get-started (and /learn/home/get-started.md).

It is the target of the "Copy prompt for your agent" CTA on the marketing-site hero (fern-api/marketing-site#453). The homepage prompt stays short and evergreen ("read this page and follow it"); this page owns the actual onboarding flow, so wording changes land here rather than in the site button:

1. Ask the user which product to set up (Docs, SDKs, both) and whether they have an OpenAPI spec.
2. Docs: install the fern-docs skill, follow the Docs quickstart .md.
3. SDKs: follow the SDKs quickstart .md; Docs first when both.
4. Report what was created and the next command.

hidden: true keeps it out of the Home sidebar. fern check and Vale pass.

Preview: https://fern-preview-devin-1789416771-agent-onboarding-page.docs.buildwithfern.com/learn/home/get-started

Link to Devin session: https://app.devin.ai/sessions/bea0c89bf3d94671bc7936888af6d6f4
Open in Devin Desktop: https://app.devin.ai/desktop/session/bea0c89bf3d94671bc7936888af6d6f4?variant=devin
Requested by: @matlegault

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[github-actions]

🌿 Preview your docs: https://fern-preview-devin-1789416771-agent-onboarding-page.docs.buildwithfern.com/learn

Here are the markdown pages you've updated:

• learn/home/get-started