[Fundamentals] Update add-site.mdx (#24213)

Reported in the Community 
The "Add a site" title that hasn't changed is confusing
https://community.cloudflare.com/t/cant-add-site-for-domain-trasnfer/820839

Author: ngayerie

src/content/docs/fundamentals/manage-domains/add-site.mdx

@@ -1,5 +1,5 @@
 ---
-title: Add a site
+title: Onboard a domain
 pcx_content_type: tutorial
 
 ---
@@ -18,7 +18,7 @@ To use Cloudflare as a reverse proxy but maintain your DNS provider, refer to [p
 
 [Disable DNSSEC](/dns/dnssec/#disable-dnssec) at the registrar where you purchased your domain name to avoid connectivity errors when you change your nameservers.
 
-## 1. Add site in Cloudflare
+## 1. Onboard a domain in Cloudflare
 
 1. Log in to the [Cloudflare dashboard](https://dash.cloudflare.com/login).
 2. Select **Add** > **Connect a domain**.