ci: cleanup concurrency rules

This addresses issues were CI jobs would wrongly get cancelled due to
sharing the concurrency group as other workflows.

For example:
- Twister runs being cancelled if both contributor submitting PRs named
  their branch the same (often, `main` :))
- The scheduled doc-build systematically kills any ongoing
  "on_pull_request_target" workflow runs.

Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>