ci: doc: refactor doc publish

Publish action has been refactored into a workflow_run based action.
The action is now triggered after a successful doc-build workflow run
(only on main/release branches). The artifacts are downloaded from the
build run and published accordingly. Publish code has been simplified a
bit and PDF upload has been added.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>