Blade: Variables like index only available in Antlers

[duncanmcclean]

On the collection/taxonomy docs, it mentions the fact there should be an index variable available.

However, the index variable (along with a couple others) is only available in Antlers loops, not in Blade loops.

Laravel Blade has its own $loop variable which can be used to get stuff like the index instead.

I'm not sure where/how, but we should document that things work differently in Antlers vs Blade.