Document `GIT_STRATEGY` and `GIT_DEPTH`
What does this MR do?
Documents GIT_STRATEGY and GIT_DEPTH. Also fixes the TOC since the anchors weren't working.
Are there points in the code the reviewer needs to double check?
The TOC and the location of the new entries.
Why was this MR needed?
What are the relevant issue numbers?
https://gitlab.com/gitlab-org/gitlab-ci-multi-runner/merge_requests/188
Screenshots (if relevant)
