Rename repo to "learn" or "Learn-OpenAPI"

[handrews]

@MikeRalphson suggested perhaps renaming this repo on Slack yesterday, suggesting "learn" as that is the site name. I'd suggest "Learn-OpenAPI" to make the repo name more distinctive and less likely to collide when someone forks it ("Documentation" is about as generic and collision-likely as they come). "Learn-OpenAPI" also matches the capitalization of "OpenAPI-Specification", "Documentation", etc.

[darrelmiller]

Done. Renamed to learn.openapis.org to match the domain. Decision made by those present in the Oct 12th, 2023 TDC call.