Add docs for foreach-loops concept

[ErikSchierboom]

The foreach-loops concept is listed in the config.json's concepts key, but doesn't have its corresponding files in the concepts/foreach-loops directory.

[BNAndras]

Caused by #199 which removed the foreach-loops concept files and fixed by #208 which dropped it from config.json.

This concept seems to have been replaced by for-loops instead.