Add H3 menu entries for leftnav:

 - Add H3 formatting and entries to leftnav, to docs css.

Docker-DCO-1.1-Signed-off-by: Jason Giedymin <jasong@apache.org> (github: JasonGiedymin)
Merge pull request #6567 from JasonGiedymin/add-h3

Add H3 menu entries for leftnav