docs on :for directive are missing information on index modifiers

[trashhalo]

https://surface-ui.org/template_syntax

:for - Iterates over a list (generator) and renders the content of the tag (or component) for each item in the list.

But reading the test cases it clearly supports a handful of modifiers for accessing indexes. It would be nice if that was listed in the documentation and a few examples provided.

https://github.com/surface-ui/surface/blob/main/test/surface/integrations/directives_test.exs#L533-L535