As per the parent ticket, we need to promote the MediaInfo RDF format to stable as part of the process of moving WCQS to production. In order to do so, we need a stable interface policy for Commons MediaInfo RDF (which doesn't currently exist).

Note: The Wikidata Stable Interface policy notes that changes "will be available for testing at least two weeks before deployment on https://test.wikidata.org/". We do currently have a https://test-commons.wikimedia.org/; however it is planned to to be deleted in T213295. We need to decide whether we should keep Test Commons for this purpose, use Beta Commons instead, or remove this clause from the stable interface policy. [Update: we will remove this clause].

Acceptance Criteria:

• Write a new policy based on the existing Wikidata policy and post it on Commons
• Remove the clause about Test Commons since it's not applicable to RDF
• From @dcausse's comment in the parent task: Remove MediaInfo from https://www.mediawiki.org/wiki/Wikibase/Indexing/RDF_Dump_Format and move to be maintained on its own page in https://www.mediawiki.org/wiki/Extension:WikibaseMediaInfo/RDF_mapping to avoid future confusions with the two policies