FROZEN: Add build-time OpenAPI generation doc

[captainsafia]

EDIT by @Rick-Anderson

All commit and suggestions should be made in #33361 #33361 will merge into this PR ----------- END of edit -----------

Adding a new page to document our build-time OpenAPI document generation capabilities.

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Internal previews

📄 File	🔗 Preview link
aspnetcore/fundamentals/openapi/aspnetcore-openapi.md	aspnetcore/fundamentals/openapi/aspnetcore-openapi
aspnetcore/fundamentals/openapi/buildtime-openapi.md	aspnetcore/fundamentals/openapi/buildtime-openapi