Add api.url configuration documentation

[mintlify-development]

Summary

• Adds documentation for the new api.url configuration option
• When set to full, the full base URL is shown on every endpoint page

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Note

Low Risk
Low risk documentation-only change with no code or behavior modifications.

Overview
Documents a new api.url configuration option in api-playground/overview.mdx that lets docs authors choose whether endpoint headers display the full base URL (full) versus just the relative path (default).

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