feat: add documentation for upstream parameter 'prompt'

[David-Wobrock]

Relates to the feature added here ory/kratos#3276

Related Issue or Design Document

• feat: handle 'prompt' upstream parameter during OIDC flow kratos#3276
• Add support for "prompt=select_account" when using social sign-in kratos#2709

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• 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 works.
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