Add docs URL to the output of k6 --help

[sniku]

I suggest we add the following sentence to the output of k6 --help:
"Full CLI documentation available at https://docs.k6.io/docs/options"

[imiric]

I see you tried man k6 before --help. ;)

On that note, what do you think about distributing a man page as well? I'm not sure how easy it would be to convert the current documentation, but maybe something to consider for the upcoming documentation restructuring. It would be great for the repo to be the single source for docs, which can be built to several targets.

[na--]

I'm very much in favor of having man pages for k6, especially since we distribute some os packages anyway, so feel free to create one 😄 Though it's going to be very tricky to reuse the same text for our man pages and the real docs... And it's likely that man pages would come much later than the new docs, given how they'll probably be tied together with the configuration restructuring of k6...