Add documentation for flags functionality

The flags functionality is related to the following pages:

https://doc.nuxeo.com/userdoc/browse/

https://doc.nuxeo.com/userdoc/favorites/

https://doc.nuxeo.com/nxdoc/nuxeo-retention-functional-overview/

(also related to coldstorage if we create that section in the future)

The flags functionality consists  of adding a new (hidden by default) column to:

• search results
• document list

Which is named nuxeo-data-table-column or 'flag'.  

For now, the flag column displays visually if the document is:

• under cold storage
• under legal hold
• under retention
• favorite