I’m new to Dovecot and email administration in general, but the above documentation talks about using IMAP ACLs. I just scrolled through some of the GitHub issues like this one I see the Mail app using the same mechanism too:
This is not a solution. Yes, the mail app is only a client, but it should enable users to configure ACLs on mailboxes, like the ACL-plugin with e.g. RoundCube. Iff the IMAP-server has the mailbox sharing capability.