docs: add favicon / robots.txt

In the past, these files of the buildroot website weren't under git
version control, but rather were static files added to the git checkout
used by the webserver.

This is no longer the case, so add them to git - And at the same time
use the Buildroot logo for the favicon, rather than reusing the busybox
one.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>