Docs: move Time Tracking to correct location

Problem to solve

Move time tracking doc to correct location.

The following discussion from gitlab-foss!32894 (comment 216785601) should be addressed:

I was thinking about moving the doc to its correct location, which would be /user/time_tracking/index.md, as well as moving the images to /user/time_tracking/img/, but since there's a link from the UI to the docs, we need to do it in a non-docs- branch. Could you pls follow up on this?

Who can address the issue

@msedlakjakubowski

Other links/references

Follows https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/32894