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. https://gitlab.com/gitlab-org/gitlab/-/issues/326252 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.
issue