Add troubleshooting entry about OpenAPI base URL errors

What does this MR do?

Adds an entry in the troubleshooting section in order to explain how to resolve the issue when there is not enough information about the target API when using OpenAPI. The API Fuzzing analyzer also allows specifying the target API using different options (e.g. environmental variables, files), also the OpenAPI specification allows optionally specifying target API information. This doc update provides information on how to resolve the issue when it happens.

Related issues

• #329143 (closed)

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