Create API docs page for `Microsoft.OpenApi` package

[captainsafia]

I'm currently working on adding documentation on using the new Microsoft.AspNetCore.OpenApi package and am finding that I need to reference Microsoft.OpenApi types frequently. It would be great if there was a docs site for the package so that dependencies can link to them in their docs.

Let me know if this already exists!

[martincostello]

This documentation does appear to exist, however it hasn't been updated for Microsoft.OpenApi v2.