docs: Add page about IMAP-only configuration

docs/tutorials/imap-only.md

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+# Using maddy with an external SMTP server
+
+It is possible to use maddy as an IMAP server only
+and have it interface with external SMTP server using
+standard protocols.
+
+Here is the minimal configuration that creates
+a local IMAP index, credentials database and IMAP
+endpoint:
+```
+# Credentials DB.
+pass_table local_authdb {
+    table sql_table {
+        driver sqlite3
+        dsn credentials.db
+        table_name passwords
+    }
+}
+
+# IMAP storage/index.
+imapsql local_mailboxes {
+    driver sqlite3
+    dsn imapsql.db
+}
+
+# IMAP endpoint using these above.
+imap tls://0.0.0.0:993 tcp://0.0.0.0:143 {
+    auth &local_authdb
+    storage &local_mailboxes
+}
+```
+
+To accept local messages from an external SMTP server
+it is possible to create an LMTP endpoint:
+```
+# LMTP endpoint on Unix socket delivering to IMAP storage
+# in previous config snippet.
+lmtp unix:/run/maddy/lmtp.sock {
+    hostname mx.maddy.test
+
+    deliver_to &local_mailboxes
+}
+```
+
+Look up documentation for your SMTP server on how to make it
+send messages using LMTP to /run/maddy/lmtp.sock.
+
+To handle authentiation for Submission (client-server SMTP) SMTP server
+needs to access credentials database used by maddy. maddy implements
+server side of Dovecot authentication protocol so you can use
+it if SMTP server implements "Dovecot SASL" client.
+
+To create a Dovecot-compatible sasld endpoint, add the following configuration
+block:
+```
+# Dovecot-compatible sasld endpoint using data from local_authdb.
+dovecot_sasld unix:/run/maddy/auth-client.sock {
+    auth &local_authdb
+}
+```