Document + doctest using gateway injection for SPIRE/SPIFFE CSI integration

Right now the SPIRE integration doc explains how to set up SPIRE and CSI volume injection for sidecars, but notably doesn't show how to do that for gateways, which can also support injection templates.

Docs should be updated to show CSI injection for gateways as well, and a doctest for the same would ensure the docs stay relevant.

Semi-related: https://github.com/istio/istio/pull/53252