Update direct integration guide with new catalog endpoint
v1.2 of the Enterprise Direct Integration Guide describes a deprecated endpoint for getting information about customer catalogs. Instead of /enterprise/v1/catalogs we should use /enterprise/v1/enterprise-catalogs.
References to /enterprise/v1/catalogs are mostly in section 7 of the guide, but we need to check the entire guide.
This came up when we had some confusion with Genpact, who are integrating via EdCast. They have enterprise customer catalogs set up but were following documentation trying to access the old-style endpoints. As a result their API call did not return any catalogs and they had to contact our customer success team.