Add variables property to docs.json reference

[mintlify]

Summary

• Documents the new variables property in docs.json
• Variables use {{variableName}} syntax and are replaced at build time
• Variable names support alphanumeric characters, hyphens, and periods

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Note

Low Risk
Low risk documentation-only change; no runtime behavior or configuration parsing is modified.

Overview
Adds documentation for a new docs.json variables object, including {{variableName}} substitution syntax, examples for defining/using variables in MDX, and notes on allowed variable-name characters plus sanitization/undefined-variable behavior.

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