[RTA/DPA BW tests]: Add warning to BW test pages about auto deletion and section to docs about it

Can be tested after lhcb-core/LHCbPR2HD!315 (merged) is merged.

Adds a warning to the dashboard telling the reader when the result is to be deleted.

Also points to the documentation, where there is a new section about this auto deletion, and some instructions about how to preserve a bandwidth test on https://lhcb-reference-bandwidth-tests.web.cern.ch/

The warning will look something like

And the new docs section looks like

TODO:

• Create the website via webEOS request,
• Test build the pages with the warning,
• Test the doc build.

Will need to be cherry-picked (-the last commit) to master -> !5195 (merged)