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 the things
Updated content from Nuritzi

Filling in stubs from the earlier commit, update other pages

[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]