ci: add github actions for pull requests automated checks

[guilhem-sante]

This PR add two basic GitHub Actions to make small CI automations:

• CI: proceed to some automated checks when called (can only be called manually or by other GitHub Actions).
• PR: triggered when a Pull Request is made or edited it trigger the CI GitHub Action

Side-Notes

I've also added a badge on the README.md to indicate the current CI status of the main branch.

[guilhem-sante]

The Jobs of the CI are expected to fail.
The full completion of the CI is a separate concerns that should be addressed in a separate PR.