remove catalog endpoint mentions

[philvarner]

Related Issue(s):

• Uncertainties around /catalogs/{catalog_id} #398

Proposed Changes:

1. Remove mention of catalogs endpoint and catalog hierarchies, as these sections were incomplete and confusing. This text has been moved to:

• How to return dynamic sub-catalogs stac-api-extensions/best-practices#4 (comment))

for future use.

PR Checklist:

• This PR has no breaking changes.
• This PR does not make any changes to the core spec in the stac-spec directory (these are included as a subtree and should be updated directly in radiantearth/stac-spec)
• I have added my changes to the CHANGELOG or a CHANGELOG entry is not required.