Added that local usernames can't be used in Hestia. (#5134)

Hestia uses Linux system, so local users already present can't be added to Hestia.

docs/docs/introduction/best-practices.md

@@ -5,6 +5,7 @@ Managing a server is not easy. Here are some best practices you should try to ad
 ## Use a regular user
 
 Before adding any web or mail domain on your server, you should create a regular user. To do this, you can refer to our [User Management Guide](../user-guide/users#adding-a-user).
+Remember, Hestia uses Linux system for User Management, so any user you already created can't be added to Hestia.
 
 ## Enable two-factor authentication (2FA) for the _admin_ user
 

docs/docs/user-guide/users.md

@@ -8,6 +8,10 @@ To manage users, log in as an **administrator** and navigate to the **Users <i c
 2. Fill out the fields.
 3. Click the **<i class="fas fa-fw fa-save"></i> Save** button in the top right.
 
+::: warning
+Hestia uses Linux system for User Management, so any user you already created locally can't be added to Hestia.
+:::
+
 ## Impersonating a user
 
 1. Hover over the user you want to login as.