Enable "proper" PR flow when there is a second contributor

[AFulgens]

Context

PR flow with a pipeline after approval is not feasable with one contributor, because you cannot approve your own PRs [1].

Up until then YOLO, I guess.

To do

When there is a second contributor to the project, consider the following:

• add trigger workflow_call: to main.yml
• add pr-reviewed.yml:

name: PR approved
on:
  pull_request_review:
    types:
      - submitted

jobs:
  pr-approved:
    if: github.event.review.state == 'APPROVED'
    name: PR has been reviewed and approved, run main pipeline before merge
    uses: ./.github/workflows/main.yml

• enable mandatory review for PRs.

[1] https://github.com/orgs/community/discussions/6292