Add documentation for for-loops

[plsnp]

At the moment i only find documentation for for-loops at https://ginger.tobiasdammers.nl/guide/getting-started/ under heading "Ginger Syntax: The Basics.". I would like to loop over a dictionary. Sometimes only keys are needed, sometimes only values, sometimes both. Maybe there could be a documentation page like twig's for

[tdammers]

It would belong somewhere in https://ginger.tobiasdammers.nl/guide/syntax/statements/, but does in fact not exist yet.

If you feel like it, feel free to send a patch - the documentation is generated from https://github.com/tdammers/ginger/blob/master/doc/syntax/statements.markdown and forwarded to the ginger website from there; but if you don't feel qualified, ping me and I'll add it myself.

[plsnp]

I looked into the code and from what i understood looping over 1. values OR 2. values + keys are supported. Looping over only keys is not supported. And i also didn't see similar syntax like twig with pipe symbol (maybe i overlooked somewhere). https://twig.symfony.com/doc/3.x/tags/for.html#iterating-over-keys

Slice is supported though with slightly different syntax.

Could you confirm if those observations are correct ?