Document how to get the statistics we need out of Jenkins

For SponsorS reports we need:

• average daily number of topic branches we’ve built at least one ISO image from (note that it’s not exactly what I asked for #9939 (closed), sorry);
• number of test suite runs per release cycle.

For #9939 (closed), bertagaz compiled close enough stats “According to the GolbalBuildStat Jenkins plugin”, which is great, but it is not documented how someone else would do it themselves. A very rough doc draft in the sysadmin team Git repo would be good enough IMO, as long as at least I am able to follow it to successfully compile such stats.

This process will also allow us to identify any manual steps in this process that should be automated, if any (but automating them is not part of this ticket, of course).

Parent Task: #5288 (closed)

Original created by @intrigeri on 10507 (Redmine)