doc: update documentation to use the --shield west build command arg

Update zephyr-app-commands to generate "west build --shield ..." for west
invocations using shields while still generating "cmake .. -DSHIELD=..."
if the tool is set to "cmake".

Updated the various shield documentation pages to recommend using
"--shields" over "-DSHIELD" to match the results of the corresponding
zephyr-app-commands code snippets.

Add proper support to zephyr-app-commands for specifying multiple shields
(using a comma-separated list) and fix a few invocations to use this.

Signed-off-by: Henrik Brix Andersen <hebad@vestas.com>