Document xml request creation for OpenAPI

What does this MR do?

Document an enhancement in OpeanAPI reader. The OpenAPI reader produces content targeting specific media types. Now is able to produce application/xml media type by following OpenAPI recommendations on how to produce it.

Related issues

• #344545 (closed)

Author's checklist

• Consider taking the GitLab Technical Writing Fundamentals course.
• Follow the:
  • Documentation process.
  • Documentation guidelines.
  • Style Guide.
• Ensure that the product tier badge is added to topic's h1.
• Request a review based on:
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  • The associated Technical Writer.

Review checklist

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• If the content requires it, ensure the information is reviewed by a subject matter expert.
• Technical writer review items:
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  • Ensure the appropriate labels are added to this MR.
  • If relevant to this MR, ensure content topic type principles are in use, including:
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• Ensure a release milestone is set.