Provide a link icon for subsections

The old docs.go.cd used to show a link icon for each subsection on hover. Clicking on this would provide the provide the link to the subsection in the url. It would be nice to have that for the new docs.

The issue names no files or tests. Start by locating how documentation subsection headings and anchors are rendered, then compare the current output with the described old docs.go.cd behavior. Done means subsection headings show a hover link icon and clicking it produces the subsection URL.

Written by the indexing model from the issue text.