Add README for multi-source docs example

[devin-ai-integration]

Summary

Adds a multi-source/README.md that documents the multi-source docs example with:

• Screenshots of the homepage and a sub-site page
• Project structure overview (all 6 sub-projects mapped to their URL paths)
• Full theme.yml showing what the global theme provides (colors, CSS, logo)
• Theme upload workflow (fern docs theme export → fern docs theme upload)
• Complete sub-site docs.yml example (Sunflower) showing how minimal the config is
• Publishing instructions
• Features summary table

Review & Testing Checklist for Human

• Verify the README renders correctly on GitHub at multi-source/README.md
• Check that screenshots display properly in the README

Notes

The screenshots are committed as PNGs in multi-source/assets/. They show the live site at https://multi-source.docs.buildwithfern.com.

Link to Devin session: https://app.devin.ai/sessions/a308a5b20cf34d64b9b446fa5de28a3f

[devin-ai-integration]

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[github-actions]

🌿 Preview your docs (multi-source/seeds): https://fern-docs-examples-preview-devin-1777841599-multi-source-readm.docs.buildwithfern.com/seeds

[github-actions]

🌿 Preview your docs (multi-source/homepage): https://fern-docs-examples-preview-devin-1777841599-multi-source-readm.docs.buildwithfern.com

[github-actions]

🌿 Preview your docs (multi-source/seeds-tomato): https://fern-docs-examples-preview-devin-1777841599-multi-source-readm.docs.buildwithfern.com/seeds/tomato

[github-actions]

🌿 Preview your docs (multi-source/seeds-sunflower): https://fern-docs-examples-preview-devin-1777841599-multi-source-readm.docs.buildwithfern.com/seeds/sunflower

[github-actions]

🌿 Preview your docs (multi-source/nursery): https://fern-docs-examples-preview-devin-1777841599-multi-source-readm.docs.buildwithfern.com/nursery

[github-actions]

🌿 Preview your docs (i18n): https://fern-docs-examples-preview-devin-1777841599-multi-source-readm.docs.buildwithfern.com

[github-actions]

🌿 Preview your docs (multi-source/greenhouses): https://fern-docs-examples-preview-devin-1777841599-multi-source-readm.docs.buildwithfern.com/greenhouses

[github-actions]

🌿 Preview your docs (docs-starter): https://fern-docs-examples-preview-devin-1777841599-multi-source-readm.docs.buildwithfern.com

[github-actions]

🌿 Preview your docs (versioning): https://fern-docs-examples-preview-devin-1777841599-multi-source-readm.docs.buildwithfern.com