docs: add x-jsonschema-if extension page

[baywet]

Summary

• Adds one OpenAPI extension registry page for a JSON Schema compatibility extension.
• Documents that the keyword is serialized as x-jsonschema-* when targeting OpenAPI versions where the JSON Schema keyword is not directly available.

OpenAPI.NET evidence

• https://github.com/microsoft/OpenAPI.NET/blob/main/src/Microsoft.OpenApi/Models/OpenApiSchema.cs exposes PropertyNames, DependentSchemas, If, Then, and Else schema members.
• https://github.com/microsoft/OpenAPI.NET/blob/main/src/Microsoft.OpenApi/Reader/V3/OpenApiSchemaDeserializer.cs maps the corresponding OpenApiConstants extension names into those schema members during OpenAPI v3 deserialization.

Validation