docs: CSS changes broke anchor link positions

Created by: slimsag

Click this link: https://docs.sourcegraph.com/admin/updates/pure_docker#v3-12-5-v3-13-2-changes

You will not end up in the right position, the navigation bar overlaps the heading.

This worked previously and is a regression, see https://sourcegraph.slack.com/archives/CCVUFTEFN/p1584469686014900?thread_ts=1580500017.007500&cid=CCVUFTEFN for more information.