doc: really get rid of Documentation/ directory

Commit 656d8da9 (doc: Remove duplicated documentation directory) got
rid of most of Documentation/. But there's still an obviously useless
.gitignore left behind.

Also, there's a copy of the linux kernel's net/ethernet.txt binding
imported from v5.0, while the existing one in doc/ is from 4.0-rc1. So
replace the latter by the former, and making Documentation/ finally
empty.

Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Reviewed-by: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>