Document RFC9727 usage for discovering OADs

[handrews]

RFC 9727 api-catalog: A Well-Known URI and Link Relation to Help Discovery of APIs is the answer to several OAS issues, but as we do not wish to endorse one specific mechanism in case others become popular, it should be documented on the learn site.

Addresses:

• Linking API to OpenAPI and documentation OpenAPI-Specification#724
• RFC 5785 conventions & suggested OpenAPI document locations OpenAPI-Specification#1851
• Support discovering OpenAPI document from Link header? OpenAPI-Specification#2128