Is your feature request related to a problem? Please describe.

This feature would be extremely useful in general, but is a requirement for the following common use case:

• A markdown collection has existing entries
• Images in the bodies of those files come from multiple directories
• The best the CMS can do currently is show one configured directory in the media library for the entire collection
• Images that are not sourced from the configured directory aren't visible
• Even if we made it so they were visible, the directories they point to are still inaccessible from the media library

Describe the solution you'd like

For sites of any complexity, Eg., large existing open source docs sites, which Open Authoring was created for, navigating directories in the media library is a necessity.

From #1046 (comment):

Requirements

• Allow multiple folders to be used
• Allow nested folders
• Allow users to create folders

Proposed approach

• Keep accepting a single media folder in the config, as we do currently
• Show a list of folders found within the media folder on the left side of the media library
• Use breadcrumbs to track position in the hierarchy
• Allow new folders to be created via an input after the last folder in the list with placeholder "new folder"
• Allow folders to be deleted via a button that shows on hover of each folder
• Folders must be empty to delete

Describe alternatives you've considered

No support.

Additional context

Replaces #1046.