Add input document for OpenAPI

[jonaslagoni]

Reason/Context

We currently only have one document describing a single input type (JSON Schema), I think we need to have one for OpenAPI as well.

This document should describe the different use-case of OpenAPI similar to how https://modelina.org/docs/usage#generate-models-from-swagger-20-documents and https://modelina.org/docs/usage#generate-models-from-openapi-documents explain it. We then need to adapt the usage document similarly to how JSON Schema refers to the new doc: https://modelina.org/docs/usage#generate-models-from-json-schema-documents

[github-actions]

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