Document localhost OpenAPI previews

[hamedrabah]

Documentation changes

Documents the existing local OpenAPI workflow:

• Reference a specification served from localhost in docs.json.
• Run mint dev with --local-schema.
• Use the same flag with mint validate.
• Keep HTTPS as the production requirement.

Closes #617

Validation

• mint validate --telemetry=false
• mint broken-links --files api-playground/openapi-setup.mdx --telemetry=false
• git diff --check

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Note

Low Risk
Documentation-only change with no runtime, auth, or data-handling impact.

Overview
Adds a Use a localhost specification section to the OpenAPI setup guide so developers can preview API docs against a spec served from their machine during local work.

The new content shows pointing navigation.openapi at a http://localhost:… URL, running mint dev --local-schema so the CLI can fetch that spec over HTTP, and the same --local-schema flag with mint validate when checking config without a dev server. It also states that production OpenAPI URLs must use HTTPS.

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[ethanpalm]

Thanks for creating this PR, @hamedrabah. Appreciate you taking the time to improve the Mintlify docs.