The main page, Dashboard, contains links to services such as GeoServer, MapStore2, pg_tileserv, as well as tutorials, and documentation.
It also has links to demo Apps, as well as the control panel login page.
The Getting Started page contains sequential tutorials for loading data and other components.
The Docs page contains the full documentation.
Remove or Replace
You can remove the Welcome app, or replace it with your content by removing the contents of /var/www/html:
rm -rf /var/www/html/*