Index role / directive (similar to sphinx)

[rowanc1]

Which area is this feature request for?

Website User Interface, Command Line Tools

Describe the feature you'd like to request

https://www.sphinx-doc.org/en/master/usage/restructuredtext/directives.html#index-generating-markup

### Inner Product and Norm

```{index} single: Vectors; Inner Product
```

```{index} single: Vectors; Norm
```

Describe the solution you'd like

No response

Describe alternatives you've considered

No response