source/what-is-linux.html: Re-factor screenshot of Unix family tree

Signed-off-by: Alan Orth <alan.orth@gmail.com>
source/what-is-linux.html: Add slide for Debian

Signed-off-by: Alan Orth <alan.orth@gmail.com>
source/what-is-linux.html: Add section on Ubuntu family tree

Signed-off-by: Alan Orth <alan.orth@gmail.com>
source/what-is-linux.html: Fix broken section

Signed-off-by: Alan Orth <alan.orth@gmail.com>
source/what-is-linux.html: Add slide with epic mainframe picture

Signed-off-by: Alan Orth <alan.orth@gmail.com>