docs: add multi-source docs page

[devin-ai-integration]

Summary

Adds a new documentation page for the multi-source docs feature, which lets multiple teams publish to the same custom domain from independent repositories.

Changes

• New page: fern/products/docs/pages/preview-publish/multi-source.mdx — covers:
  • Enterprise-only warning at the top
  • How multi-source: true on instances enables basepath-aware publishing
  • How global themes (fern docs theme export/upload/list) keep branding consistent across repos
  • Dashboard multi-source settings for default path and search behavior (hierarchical vs unified)
  • Full setup guide with Steps, Tabs, and a worked example (homepage + 3 teams with 4 repos)
• Navigation: Added the page to the "Preview & publish" section in docs.yml
• Site-level settings: Added instances.multi-source ParamField to the instances configuration reference
• Changelog: Added 2026-04-30.mdx entry for the feature
• Screenshot: Dashboard multi-source settings screenshot (text updated to "Multi-source settings")

Review & Testing Checklist for Human

• Verify the multi-source page renders correctly in the preview deployment — check Steps, Tabs, Warning, CardGroup example, image, and ParamField components
• Confirm the screenshot of dashboard multi-source settings accurately represents the current UI
• Check that all internal links resolve (/learn/docs/customization/global-themes, /learn/docs/preview-publish/multi-source, etc.)
• Verify the changelog entry appears on the changelog page

Notes

• The page is placed under "Preview & publish" since multi-source is an extension of the publishing workflow.
• The instances.multi-source property replaces the deprecated experimental.basepath-aware flag (added in CLI 5.2.0).
• Cross-references the existing Global themes page rather than duplicating theme setup instructions.
• The example section demonstrates the recommended "homepage + 3 teams" pattern with 4 separate fern/ projects.

Link to Devin session: https://app.devin.ai/sessions/b3a9f06371964eb780a68b4e88cb81ab

[devin-ai-integration]

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[github-actions]

🌿 Preview your docs: https://fern-preview-devin-1777565083-add-multi-source-docs.docs.buildwithfern.com/learn

Here are the markdown pages you've updated:

• learn/docs/changelog/2026/4/30
• learn/docs/getting-started/project-structure
• learn/docs/configuration/site-level-settings
• learn/docs/preview-publish/multi-source-docs