move Python-specific help links to kernel_info

help_links in kernel_info were already in the msg spec, but not implemented.

Python-specific docs are now linked to the Python version.

closes #7323

ping @ellisonbg
Arrays have both `.map()` and `.forEach()`, and it looks like the only difference is that map builds an array of the returned values, while forEach doesn't, which suggests that forEach is what we want here. Hopefully modern JS interpreters are smart enough to see that the return value isn't used and optimise it out, though.