ci: doc-publish: Do not run for pull requests
ci: doc-publish: Do not run for pull requests

GitHub Actions may trigger the `workflow_run` event type when the
pull request source branch name matches one of the branches specified
under `branches:`.

This commit updates the documentation publish workflow to skip when the
preceding event type is `pull_request`, in order to prevent pull
request documentation from being uploaded as main documentation.

Signed-off-by: Stephanos Ioannidis <dc76e9f0c0006e8f919e0c515c66dbba3982f785@stephanos.io>

zephyrproject-rtos/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,galak/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,finikorg/zephyr