_redirects: Add _redirects file

Add 301 perm redirect for Weston release tarballs from 10.0.1
until 13.0.1. These should be all removed in a sub-sequent update.

For 10.0.2 and 10.0.3 there are no tarballs at all (never being uploaded
to releases/ directory) so this redirect should fix that as well.

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>