binding-template.yaml: Improve 'include:' docs and mention base.yaml

Explain how 'include:' works in some more detail and mention base.yaml,
along with an example of how it can be used.

This was adapted from
https://github.com/zephyrproject-rtos/zephyr/pull/19846

.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>