Cleanup: Remove exported docs, flatten/simplify directory hierarchy
    
    This commit is in response to recent Pull Requests to the i3-sig/docs
    repo. It removes the Ideas.md and Kickstarts/README.adoc files from this
    repo. They now exist in the docs repo:
    
        https://pagure.io/i3-sig/docs/pull-request/4
    
        https://pagure.io/i3-sig/docs/pull-request/6
    
    I also removed the old LXDE/XFCE kickstarts since they are unused. I
    moved the `flat-i3.ks` kickstart to the root directory, since it is the
    only file remaining.
    
    cc: @nasirhm @x3mboy @defolos
    
    Signed-off-by: Justin W. Flory (he/him) <git@jwf.io>