docs: add return_to query parameter to OAS Verification Flow for Native Apps

[David-Wobrock]

The idea would be to re-generate the Kratos SDK, so that one can specify a return_to URL when creating a Native Verification Flow using the SDK.

Related issue(s)

Checklist

• I have read the contributing guidelines.
• I have referenced an issue containing the design document if my change
  introduces a new feature.
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• I have added tests that prove my fix is effective or that my feature
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• I have added or changed the documentation.

Further Comments

[aeneasr]

I checked and the return_to parameter is not being used anywhere in the verification flow, so this change has no effect. We would instead need to implement return_to support.

What's your use case?