[CIMANAGE-29] Set top level description on Sandbox

[vvalderrv]

Why: We can insert link to the guide [0], so anybody visiting main Sandbox page [1] can learn how to (ask to) use it.

How: Perhaps by creating a default view, the same as the current default (All), but with some friendly message in the description. See the production [2] for and example of a default view with a message.

Caveat: Sandbox gets wiped out during weekends, so the edit should probably be done in global-jjb around here [3].

I do not know how to set a description there, that is why I am opening a ticket, instead of contributing directly.

[0] https://docs.releng.linuxfoundation.org/en/latest/jenkins-sandbox.html
[1] https://jenkins.fd.io/sandbox/
[2] https://jenkins.fd.io/
[3] https://github.com/lfit/releng-global-jjb/blob/9bbd8bf9729d82dce060cacbb28e684d59ef1b4a/shell/jenkins-sandbox-cleanup.sh#L26-L28