x-mint

[ethanpalm]

Documentation changes

This PR documents the x-mint extension

• Info on how to use the x-mint extension to override metadata and add content to an endpoint page

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[ethanpalm]

Since the x-mint features are merging before the navigation features, I am updating this PR to only have the x-mint changes and renaming it.

I created a branch openapi-nav-updates and merged the current changes to it. We'll use that when the nav changes are ready to ship 🎉