docs: add baseline ldap documentation

within the new account backends nav section.

docs/ldap-basic.nix

@@ -0,0 +1,17 @@
+{
+  mailserver = {
+    ldap = {
+      enable = true;
+      uris = [
+        "ldaps://ldap1.example.com"
+        "ldaps://ldap2.example.com"
+      ];
+      bind = {
+        dn = "cn=mail,dc=example=dc=com";
+        passwordFile = "/run/keys/ldap-bind-pw";
+      };
+      base = "ou=users,dc=example,dc=com";
+      scope = "one";
+    };
+  };
+}