docs: document reserved folder names (fern, apis, changelog)

[devin-ai-integration]

Summary

Adds documentation clarifying that the fern, apis, and changelog folders are reserved names and must be named exactly as shown. Also clarifies that pages and assets are not reserved and can use any name. Changes span three pages:

• Docs project-structure.mdx: New "Reserved folder names" section with a table and warning callout, placed after the top-level folders list. Also adds a sentence in the Multiple APIs accordion reinforcing that apis must use that exact name. Clarifies in the top-level folders list that pages and assets can use any name.
• changelogs.mdx: Adds a sentence to the intro paragraph noting the changelog folder naming requirement.
• API Definitions project-structure.mdx: Adds a sentence to the "Multiple APIs" section noting that the apis folder must use that exact name.

Review & testing checklist for human

• Verify the claim that pages and assets can truly be named anything — this is stated as fact and could mislead users if incorrect
• Verify the changelog link in the table (/learn/docs/configuration/changelogs) resolves correctly on the preview site
• Check the API Definitions project structure page to confirm the apis sentence reads naturally in context
• Consider whether the table's "Folder" and "Required name" columns are redundant — both show the same value. A simpler format (just folder + description) may be cleaner

Suggested test plan: Open each of the three preview pages linked below and read through the new/changed content to confirm accuracy and that it flows well with the surrounding text.

Notes

Requested by: Fern Support
Link to Devin run

[devin-ai-integration]

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[github-actions]

🌿 Preview your docs: https://fern-preview-bec94e2c-9f49-4c15-8fe1-de2241ed4990.docs.buildwithfern.com/learn

Here are the markdown pages you've updated:

• learn/api-definitions/overview/project-structure
• learn/docs/getting-started/project-structure
• learn/docs/configuration/changelogs