docs: document built-in Fern components for custom header and footer

[devin-ai-integration]

docs: document built-in Fern components for custom header/footer

Summary

Documents the built-in UI components available via the Fern prop in custom header and footer components, as introduced in fern-api/fern-platform#7636.

Changes to custom-header-footer.mdx:

• Added a new "Built-in Fern components" section with a beta notice, a quick-start code example, and a reference table listing all 8 available components (Logo, Search, ProductSwitcher, VersionSwitcher, LanguageSwitcher, NavbarLinks, LoginButton, ThemeSwitch)
• Renamed the "Advanced configuration" section to "Using React Hooks" and removed the accordion wrapper (only one example, so an accordion was unnecessary)
• Existing "Header with dropdown menu" example is preserved unchanged

Updates since last revision

• Removed extra explanatory sentence about null checks from the built-in components intro
• Removed the "Header with built-in components and custom links" accordion example
• Renamed "Advanced configuration" to "Using React Hooks"
• Added a <Note title="Beta"> callout marking built-in Fern components as beta and subject to change
• Removed <AccordionGroup>/<Accordion> wrapper from "Using React Hooks" section — only one example remains, so it's now shown directly under the heading

Review & Testing Checklist for Human

• Verify fern-platform#7636 is merged and deployed — this page documents behavior from that PR. If it hasn't shipped, this docs page describes unreleased functionality and should be held or gated behind a note.
• Cross-check component descriptions against actual implementation — especially ProductSwitcher ("only renders if you have multiple products configured"), LanguageSwitcher (what "language" means here is unclear from the platform code — SDK language vs. locale?), and LoginButton (does it actually appear for non-authenticated docs?).
• Preview the rendered page to verify the table, code blocks, beta note, and section formatting render correctly in the Fern docs site.

Notes

• This is a docs-only MDX change (single file). No code or config changes.
• The component API ({ Fern } prop with JSX syntax like <Fern.Logo />) matches the smoke test in the platform PR, but runtime verification requires a deployed docs site with the updated platform code.

Link to Devin run: https://app.devin.ai/sessions/ae6dd595b7ce46e8b17eb826a0f26dc5
Requested by: Fern Support

[devin-ai-integration]

🤖 Devin AI Engineer

I'll be helping with this pull request! Here's what you should know:

✅ I will automatically:

• Address comments on this PR. Add '(aside)' to your comment to have me ignore it.
• Look at CI failures and help fix them

Note: I can only respond to comments from users who have write access to this repository.

⚙️ Control Options:

• Disable automatic comment and CI monitoring

[fern-api]

Requested by: sandeep
Slack thread: View conversation

[github-actions]

🌿 Preview your docs: https://fern-preview-8c7c2335-5a6f-4849-9ba1-fb4a42bf8b0f.docs.buildwithfern.com/learn

Here are the markdown pages you've updated:

• learn/docs/customization/header-and-footer

[github-actions]

📝 [vale] _{reported by reviewdog 🐶}
[FernStyles.Headings] 'Using React Hooks' should use sentence-style capitalization.