docs: Fix up backporting instructions.
docs: Fix up backporting instructions.

Add 'HEAD' to the 'git push' command to resolve ambiguity about which
branch to push to (This may be unnecessary depending on the local git
configuration, but is never wrong, as 'HEAD' refers to the top of the
current new backport branch).

Add instructions for creating the backport PR at GitHub (I could not
get the command line tool to work).

Signed-off-by: Jarno Rajahalme <0f1ab0ac7dffd9db21aa539af2fd4bb04abc3ad4@covalent.io>
tklauser/cilium,michi-covalent/cilium,tklauser/cilium,tgraf/cilium,michi-covalent/cilium,cilium-team/cilium,tklauser/cilium,michi-covalent/cilium,cilium/cilium,cilium/cilium,tgraf/cilium,tklauser/cilium,tgraf/cilium,cilium/cilium,tklauser/cilium,tgraf/cilium,michi-covalent/cilium,cilium/cilium,cilium/cilium,michi-covalent/cilium,tgraf/cilium,cilium-team/cilium,tgraf/cilium