Add AdGuard Home environment variables page

Add an environment variables page to the AdGuard Home section of the knowledge base.

Author: schzhn

docs/adguard-home/environment.md

@@ -0,0 +1,12 @@
+---
+title: Environment variables
+sidebar_position: 5
+---
+
+This page lists the supported environment variables and their descriptions.
+
+## `ADGUARD_HOME_DEFAULT_WEB_PORT` {#ADGUARD_HOME_DEFAULT_WEB_PORT}
+
+Sets the suggested default HTTP port displayed in the installation wizard.
+
+**Default:** `80`.