The Help page for the MediaWiki REST Sandbox, https://www.mediawiki.org/wiki/Help:RestSandbox, should act as a landing page that includes

• Documentation for end users of the REST sandbox
• Contact info for escalations or filing issues
• Navigation / links to other key docs about the REST Sandbox, including:
• Link to instructions for how MediaWiki core developers should document their APIs and test API documentation display
• Link to or include instructions for MediaWiki extension developers: T426851
• Link to or include instructions for external extension developers: T426852
• Link to administrator documentation and other technical docs that explain how the Sandbox is implemented and how to maintain it (MWI team docs? Wikitech?)

It's possible that the latter items should be documented in the page for the RestSandbox as an Extension, but right now it's part of https://www.mediawiki.org/wiki/Extension:WikimediaCustomizations, and I'm not sure what that means for how/where we would put its docs. And, if that's only a temporary state, maybe for now we just use https://www.mediawiki.org/wiki/Category:OpenAPI to be able to find the rapidly-increasing number of pages?