OpenAPI-Learn and OpenAPI-Registry

[ralfhandl]

Add two entry points related to the OpenAPI specifications:

• The OpenAPI Learn site with learning material
• The OpenAPI Registry which allows registering specification extensions

Note: draft PR to allow getting feedback from OpenAPI TSC colleagues

[ralfhandl]

@lornajane @darrelmiller @earth2marsh please cross-check

[ralfhandl]

Discussed in OpenAPI TDC meeting, no objections