Document self-hosted base path configuration

[devin-ai-integration]

Document self-hosted base path configuration

Summary

Adds documentation for the NEXT_PUBLIC_BASE_PATH environment variable that allows self-hosted deployments to serve documentation from a subpath (e.g., https://example.com/docs instead of https://example.com).

This feature was added in fern-platform PR #5393 but was not documented in the user-facing setup guide. The documentation includes:

• Runtime configuration using the environment variable
• Example Docker run command
• Warning about proper slash formatting

Review & Testing Checklist for Human

• Verify environment variable name: Confirm NEXT_PUBLIC_BASE_PATH is the correct variable name for runtime configuration (I based this on code review of fern-platform PR #5393)
• Test with actual deployment: Run a self-hosted Docker container with this configuration to verify the example command works and docs are accessible at the subpath
• Validate slash formatting rules: Confirm the warning about leading/trailing slashes matches Next.js basePath behavior
• Check placement: Verify this optional step makes sense at the end of the setup flow vs. earlier in the process

Test Plan

1. Pull a self-hosted Docker image
2. Run with base path: docker run -p 3000:3000 -e NEXT_PUBLIC_BASE_PATH=/docs <image>
3. Verify docs are accessible at http://localhost:3000/docs
4. Verify docs are NOT accessible at http://localhost:3000/
5. Test with invalid formats (no leading slash, trailing slash) to confirm they fail as expected

Notes

• This is documentation-only; no code changes
• Based on code review of fern-platform PR #5393 which added this feature
• Intentionally omitted build-time configuration (BASE_PATH during image build) as that's only relevant for Fern team members, not end users deploying pre-built images
• Session: https://app.devin.ai/sessions/ef0744e80a96482a85c0f5cec53e4b4b
• Requested by: [email protected] (@dannysheridan)

[devin-ai-integration]

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[github-actions]

🚫 [vale] _{reported by reviewdog 🐶}
[Microsoft.Contractions] Use 'shouldn't' instead of 'should not'.

[github-actions]

🌿 Preview your docs: https://fern-preview-d5a3b98d-11b7-42c5-81ff-81ab40999f7e.docs.buildwithfern.com/learn

[dannysheridan]

look at who made the PR to fern-platform which inspired this PR. make that person the assignee for this PR.

[devin-ai-integration]

Assigned @coltondotio who created the original fern-platform PR #5393 that added this feature.