Remove pages we will no longer use

Mostly these are category pages we won't use with our new structure.
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 docs
[endlessm/eos-shell#3910]