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/[e-mail]
Signed-off-by: Sean Christopherson <[e-mail]>

Documentation/process/maintainer-tip.rst

···
 Some of these options are x86-specific and can be left out when testing
 on other architectures.
 
+.. _maintainer-tip-coding-style:
+
 Coding style notes
 ------------------