Documentation/process: Add a label for the tip tree handbook's coding style

Add a label for the tip tree's "Coding style notes" so that a forthcoming
KVM x86 handbook can reference/piggyback the tip tree's preferred coding
style.

Link: https://lore.kernel.org/r/20230411171651.1067966-2-seanjc@google.com


Signed-off-by: Sean Christopherson <seanjc@google.com>