Control / Set the fragment-index of {.incremental} Explicitly

[RoyiAvital]

Assume the following:

## How to Measure

::: {.incremental}
- Assume we have 2 variants of a website: `A` and `B`.
- How would we measure their design effect on user engagement?
:::

![](https://i.imgur.com/PSZggyN.png){.absolute bottom=0 .fragment .fade-in-then-out fragment-index=1}

It would be useful to be able to control the fragment-index of the items in the list in order to sync them with other items in the slide.
Of course the need is only to set the index of the first item in the list.

Remark: It was partly discussed in #3539.