add listmonk docs with smtp config details (#2265)

pages/04.applications/10.docs/listmonk/app_listmonk.md

@@ -0,0 +1,37 @@
+---
+title: listmonk
+template: docs
+taxonomy:
+    category: docs, apps
+routes:
+  default: '/app_listmonk'
+---
+
+listmonk is a self-hosted, high performance mailing list and newsletter manager
+
+## Authentication
+
+By default, there is no username-password for the admin user. In place the authentication is enforced by the reverse-proxy itself. Only the admin user chosen while installing the app can access the admin panel.
+
+More users can be given access to the panel by editing the Users and Groups permissions from the webmin or the cli.
+
+## SMTP Configuration
+
+listmonk requires an active SMTP configuration to be able to send e-mails. This can be configured in listmonk's admin interface, by going to Settings -> Settings -> SMTP
+
+### Local SMTP Configuration
+
+If you want to use your YunoHost server to send e-mails, use the following configuration:
+
+```
+Host: localhost
+Port: 25
+Auth protocol: None
+Skip TLS Verification: True
+```
+
+### Gmail, Amazon SES, Mailgun, Mailjet, Sendgrid, Postmark, e.t.c
+
+Just click on the respective buttons to get the desired configurations, and replace the relevant fields.
+
+These settings are also provided by the providers themselves.