Describe also publishing `"mediaType": "audio/mp3"` in the episode RSS for ActivityPub

[marek-lach]

Besides an episode's title, it would also seem good to describe, that an RSS entry of an ActivityPub federated podcast episode, should have a "mediaType": "audio/ogg" (or mp3, flac..) declared in its RSS entry too, as ActivityPub does have a mediaType property.

For example Pleroma already implements this, via the MediaUpload model, as does Funkwhale and it would be good to describe its use for podcasts in particular too.