Add info note on apex domains

Author: adityaoberai

src/routes/docs/advanced/self-hosting/sites/+page.markdoc

@@ -24,6 +24,13 @@ You can also configure the maximum timeout that can be set on individual Appwrit
 # Add an apex domain for Sites {% #root-domain %}
 
 Appwrite allows you to add an apex domain to your instance's configuration that you can publicly expose your Site on.
+
+{% info title="What is an apex domain?" %}
+
+An apex domain, also known as a root domain, is the highest level of a domain name without any subdomains. For example, `myapp.com` is an apex domain, while `appwrite.myapp.com` is a subdomain of the apex domain.
+
+{% /info %}
+
 To add an apex domain, you must first configure either one of the following environment variables in your `.env` file:
 
 - `_APP_DOMAIN_TARGET_A`: Set this to the IPv4 address of your server.