messaging: document file provider

[greenpau]

Instead of emailing, the file provider stores emails in a directory, e.g. {$HOME}/.local/caddy/localdb/registry

        messaging file provider localhost-file-server {
            rootdir {$HOME}/.local/caddy/localdb/registry
        }

        user registration localdbRegistry {
            #disabled
            dropbox {$HOME}/.local/caddy/localdb/registrations.json
            title "User Registration"
            code "NY2020"
            require accept terms
            require domain mx
            admin email [email protected] [email protected]
            email provider localhost-file-server
            identity store localdb
        }