Indicate current location within TOC table of contents

Problem to solve

Readme files are a great way to provide an overview of a project, however, as they grow in size, consuming their content is more difficult. #326252 (closed) introduces a table of contents making these files easier to navigate. A nice to have would be an indication wherein the file a user currently is. This was left out of the initial proposal as we currently do not have a "sticky" way to display the TOC so it would always show the top item or near top items since it is only visible near the top of the file.

Proposal

Add in a way of indicating the position within the TOC display. This would be dependant on some method of "sticky" display.