docs: prefer docs.yml subpath for self-hosted base path

[devin-ai-integration]

Summary

Reworks the "Base path" section of the self-hosted setup docs to reflect that Fern now infers the base path from the docs.yml instance subpath, so NEXT_PUBLIC_BASE_PATH is no longer required.

• Primary/recommended approach is now defining the subpath on the instance url + custom-domain in docs.yml:

  instances:
    - url: plantstore.docs.buildwithfern.com/docs
      custom-domain: docs.plantstore.dev/docs

  Notes that fern generate --docs validates url and custom-domain share the same base path, so the subpath goes on both or neither.
• NEXT_PUBLIC_BASE_PATH is now documented as an override that takes precedence over the docs.yml subpath, retaining the build-time (readOnlyRootFilesystem-compatible) vs runtime patching tradeoffs under <Tabs>.
• Updates the env-var table row for NEXT_PUBLIC_BASE_PATH to describe it as an override and sets its default to "Inferred from docs.yml subpath (else serves from /)".

Cross-reference sweep: NEXT_PUBLIC_BASE_PATH and the env-var-only base path behavior only appear on this page; the domain/multi-source pages already describe the docs.yml url/custom-domain subpath mechanism (cloud-hosted, reverse-proxy based), so no additional pointers were warranted.

Validated with pre-commit (0 Vale errors) and fern check --warnings (0 errors).

Link to Devin session: https://app.devin.ai/sessions/7772aa172d854e61b7ff24189a239940

[devin-ai-integration]

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[github-actions]

🌿 Preview your docs: https://fern-preview-devin-1783968492-self-hosted-base-path.docs.buildwithfern.com/learn

Here are the markdown pages you've updated:

• learn/docs/self-hosted/set-up

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