Make TOC go deeper than 2 levels

It would be incredibly helpful for our readers to be able to use a table of contents (TOC) that shows more than H1 and H2 in public articles. Ideally it would show down to H4 but just adding H3 would be great. Some of our documents have a lot of content at the H3 level that readers have to keep sorted in their own heads. This would also be helpful in internal articles.