Added script to validate links in our help documentation

Rather then doing all of the heavy lifting ourselves in the mallard
pages, we export to html and use existing tools to check the links
in html.
[endlessm/eos-shell#4060]
I dunno, I found it useful to have that built html around to take a look at after running the script.

But could also change this to remove the directory, or even drop the second argument and just work out of a tempdir