docs: add documentation for environment variable substitution feature

[devin-ai-integration]

Summary

Documents the substitute-env-vars setting in docs.yml that allows users to inject environment variables into their documentation content at build time. This feature already exists in the Fern CLI but was not documented.

Changes:

• Added settings.substitute-env-vars documentation to the Settings configuration section in site-level-settings.mdx
• Added the setting to the example code block
• Created a changelog entry for the feature

The documentation covers:

• How to enable the feature (substitute-env-vars: true)
• The ${VAR_NAME} syntax for referencing environment variables
• How to escape literal ${VAR} using \$\{VAR\}
• Where substitution applies (Markdown/MDX, API specs) and doesn't apply (JS files)
• Behavior differences between preview (empty string) and publish (fails on undefined)

Review & Testing Checklist for Human

• Verify the escape syntax \$\{VAR\} is correct by testing in a docs project
• Check that the anchor link /learn/docs/configuration/site-level-settings#settingssubstitute-env-vars works in the preview
• Confirm the preview vs publish behavior description is accurate (preview replaces undefined vars with empty strings, publish fails)

Suggested test plan: Enable substitute-env-vars: true in a test docs project, add ${TEST_VAR} to a page, and verify it substitutes correctly during fern docs dev and fern generate --docs.

Notes

• Link to Devin run: https://app.devin.ai/sessions/bea2fc3a13be4709978886eefad4a73d
• Requested by: Catherine Deskur

[devin-ai-integration]

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🌿 Preview your docs: https://fern-preview-f1917b5e-938a-4d87-990b-5eaf4818dbae.docs.buildwithfern.com/learn

Visual changes detected:

fern/products/docs/pages/customization/site-level-settings.mdx

Page: learn/docs/configuration/site-level-settings | Change: 76.4036448684698%

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