Add documentation for global time tracking report

Since Add global time tracking report (!108368 - merged) landed we need to update the docs!

Related Time Tracking Report for user productivity (rel... (#344002)

Implementation Guide

• Add a new doc topic below https://docs.gitlab.com/ee/user/project/time_tracking.html#view-a-time-tracking-report (file in the GitLab repo).
• Title the new topic, for example, View the global time tracking report.
• For structure, make it similar to the one above, and make sure both follow the Task doc pattern.
• In the intro before the steps explain what a global time tracking report is. Essentially the same thing as the existing time tracking report, but "at a global level". We can currently only filter by user and spent date, but the idea is to be able to slice/dice the data any way you wish. I don't think we mention that in docs, but there may be a way of us hinting at it?
• In your MR:
  • Use the "Documentation" description template, link this issue, and tag @msedlakjakubowski.
  • Assign an available Technical Writer as reviewer by creating a comment with @gitlab-bot ready <@username>.

/cc @zillemarco @msedlakjakubowski