docs(docs-yml): document settings.show-headers-in-examples

[willkendall01]

Summary

Documents the new settings.show-headers-in-examples docs.yml setting (added in fern-api/fern#17714, rendered by fern-api/fern-platform#14635).

Adds a settings.show-headers-in-examples ParamField to the site-level settings reference and the key to the settings: example block. When enabled, API Reference response examples gain a raw HTTP block — status line plus one Name: value line per declared response header — above the response body; responses with no declared headers render unchanged. Defaults to false.

The setting is not yet released: this PR can land ahead of the CLI/fern-platform changes or after, either order is safe since the docs just describe opt-in behavior.

Link to Devin session: https://app.devin.ai/sessions/292f16cc84c14e258f002d7545edb9f1
Open in Devin Desktop: https://app.devin.ai/desktop/session/292f16cc84c14e258f002d7545edb9f1?variant=devin
Requested by: @willkendall01

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[github-actions]

🌿 Preview your docs: https://fern-preview-devin-1789583487-docs-show-headers-in-examples.docs.buildwithfern.com/learn

Here are the markdown pages you've updated:

• learn/docs/configuration/site-level-settings