feat(mosquitto): Create setup-guide.md (#3212)

* Create setup-guide.md

* linting

Co-authored-by: Stavros Kois <47820033+stavros-k@users.noreply.github.com>

charts/stable/mosquitto/docs/setup-guide.md

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+# Configure password authentication
+
+## Edit chart config
+
+Change the chart's config to disable `allow_anonymous` by setting authentication to `enabled`.
+
+```yaml title="values.yaml"
+auth:
+  # -- By enabling this, `allow_anonymous` gets set to `false` in the mosquitto config.
+  # highlight-next-line
+  enabled: true
+```
+
+## Create password and config files
+
+Open the Mosquitto container shell and execute the following commands, *one by one*, in order:
+
+```sh
+cd /mosquitto/configinc
+mosquitto_passwd -c passwordfile username
+echo "password_file /mosquitto/configinc/passwordfile" > passwordconfig.conf
+```
+
+:::info These commands explained
+
+- Navigate to `/mosquitto/configinc`, the persistent storage location inside the container where the password files go
+- Create a password file. This is interactive. Change `username` to your username. It will ask you to input the password twice
+- Create a config file that points to the password file
+
+:::
+
+:::caution Restart chart
+
+Restart the chart so Mosquitto loads the new configuration file.
+
+:::