example_configs: Add Stalwart

Failed at getting Docker Mailserver and Maddy working, so I wrote a config for Stalwart Mailserver instead

example_configs/stalwart.md

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+# Stalwart Mailserver
+
+[Stalwart-mailserver](https://github.com/stalwartlabs/mail-server) is a Production-ready full-stack but simple mail server (SMTP, IMAP, LDAP, Antispam, Antivirus, etc.) written in Rust.
+
+To integrate with LLDAP, 
+
+1. Add manager user, & make sure to add to lldap_admin group for read/write permission
+   (TODO: Check if lldap_strict_readonly is enough)
+   
+3. Create mail group, add users requiring email access 
+
+4. Ensure you correctly add the following ldap settings to your Stalwart `config.toml`.
+
+## Config.toml File Sample - (only the ldap portion)
+```toml
+[storage]
+  directory = "ldap"
+
+[directory]
+  [directory.ldap]
+    base-dn = "dc=example,dc=org"
+    timeout = "30s"
+    type = "ldap"
+    url = "ldap://ldap.domain.example.org:3890"
+    [directory.ldap.attributes]
+      class = "objectClass"
+      email = "mail"
+      groups = "member"
+      name = "uid"
+      secret = "dummyStalwartSecret"
+      [directory.ldap.attributes.description]
+        0 = "displayName"
+    [directory.ldap.bind]
+      dn = "uid=manager,ou=people,dc=example,dc=org"
+      secret = "<YOUR_SECRET>"
+      [directory.ldap.bind.auth]
+        dn = "uid=?,ou=people,dc=example,dc=org"
+        enable = true
+        search = true
+      [directory.ldap.bind.filter]
+        email = "(&(|(objectClass=person)(member=cn=mail,ou=groups,dc=example,dc=org))(mail=?))"
+        name = "(&(|(objectClass=person)(member=cn=mail,ou=groups,dc=example,dc=org))(uid=?))"
+    [directory.ldap.cache]
+      entries = 500
+    [directory.ldap.filter]
+      mail = "(&(objectclass=person)(mail=?))"
+      name = "(&(objectclass=person)(uid=?))"
+    [directory.ldap.tls]
+      allow-invalid-certs = true
+      enable = false
+```