How to enable and format section numbering?

Hello! I'm working with

```bash
rst2html5 -V    
rst2html5 1.10.6 (Docutils 0.16 [release], Python 3.8.8, on linux)
```

I'm trying to get section numbering in the `x.y.z` format. While the help page mentions section numbering, the produced HTML does not seem to have anything.

What should the source RST or the incantation have to get section numberings in the output? Thanks!