mosquitto_passwd: add page (#3232)

pages/common/mosquitto_passwd.md

@@ -0,0 +1,21 @@
+# mosquitto_passwd
+
+> Manage password files for mosquitto.
+> See also `mosquitto`, the MQTT server that this manages.
+> More information: <https://mosquitto.org/man/mosquitto_passwd-1.html>.
+
+- Add a new user to a password file (will prompt to enter the password):
+
+`mosquitto_passwd {{path/to/password_file}} {{username}}`
+
+- Create the password file if it doesn't already exist:
+
+`mosquitto_passwd -c {{path/to/password_file}} {{username}}`
+
+- Delete the specified username instead:
+
+`mosquitto_passwd -D {{path/to/password_file}} {{username}}`
+
+- Upgrade an old plain-text password file to a hashed password file:
+
+`mosquitto_passwd -U {{path/to/password_file}}`