Revive old pages we bringing back

We purged a huge amount of content before the last release to
have a consistent set of pages update for endless.

Now that we have more time to update content, we are bringing a
lot back
[endlessm/eos-shell#3910]
New pages from Nuritzi

We are adding a bunch of new pages as part of a big restructure of
pages on the help center

Content not finished yet, some pages are still stubs.
[endlessm/eos-shell#3910]
Delete pages that didn't make it in new structure

A lot of category pages have been combined in the new structure so things like
net-wireless, prefs and printing got cut.

And some pages just had content that was overly technical or poorly written
[endlessm/eos-shell#3910]
New copy from Nuritzi

Written up a few weeks ago for a previous release, but we never
ended up updating the help content for that release.
[endlessm/eos-shell#3910]
Make some terms consistent

System menu, Start menu->User menu
Program->App
[endlessm/eos-shell#3910]
Remove reference to deja dup

I don't think we ship that application
[endlessm/eos-shell#3910]
Restructure page hierarchy

We now have a much simpler page hierarchy with 6 toplevel pages
which link to all the article pages.

Removed see also links
[endlessm/eos-shell#3910]
Fix broken links in help text

There were a lot of links to pages that no longer exist in pages we brought
back from the gnome-3-10 branch
[endlessm/eos-shell#3910]