Add switcher-placement documentation and changelog entry

[sbawabe]

Summary

Adds documentation for the new switcher-placement layout configuration option that allows users to control where product and version switchers appear in their documentation site.

Changes:

• New changelog entry for 2026-01-30 announcing the feature
• Updated the layout configuration section in site-level-settings.mdx with the new switcher-placement option

Review & Testing Checklist for Human

• Verify the link /learn/docs/configuration/site-level-settings#layout-configuration in the changelog resolves correctly
• Confirm the documented options (header, sidebar) and default value (header) match the actual implementation in fern-api/fern

Test Plan

1. Deploy preview and navigate to the changelog page to verify the new entry renders correctly
2. Navigate to the layout configuration section and verify the new switcher-placement ParamField appears in the correct location (after tabs-placement)

Notes

• Link to Devin run: https://app.devin.ai/sessions/08eb7b00d9ff41cca979f0111f753332
• Requested by: @sbawabe

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[github-actions]

🌿 Preview your docs: https://fern-preview-7ede95eb-7100-4cdc-bf41-6e7405548da0.docs.buildwithfern.com/learn

Here are the markdown pages you've updated:

• learn/docs/changelog/2026/1/30
• learn/docs/configuration/site-level-settings