CSP builder page needs to document existing domains

[sebbASF]

The CSP builder page at https://infra.apache.org/tools/csp.html describes how to add approved domains to the CSP, but does not state which domains are already allowed. This makes it hard for projects to work out what approvals they might need to get, and what to put in .httpaccess

It would help if the page included details of the currently allowed domains, which at time of writing are:

https://www.apachecon.com/ https://www.communityovercode.org/ https://.apache.org/ https://apache.org/ https://.scarf.sh/