Replace references to {% include %} with {% render %}

[darthmall]

LiquidJS has deprecated {% include %}; {% render %} is mostly the same (except that it does not inherit the context of the calling template), so we can just replace include with render in the docs.

Closes #1239

[zachleat]

As noted in 11ty/eleventy#2411 I do think we should add additional rows for render (and not replace include outright!) Less confusing for folks, since both are supported!