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]
Merge pull request #19 from endlessm/shell/4060

Added script to validate links in our help documentation
Sidebar matches new help docs structure

We've done a big re-layout of the structure of pages in gnome-user-docs,
update our sidebar links to point to the new top level elements and properly
match names with the actual articles
[endlessm/eos-shell#3910]