Protect against malicious usage of GitHub Actions - Use fixed commit hash instead of tag/branch refs

[Vadorequest]

Discussion at https://github.com/UnlyEd/next-right-now/discussions/223

• Fork all Actions used by NRN into our Company repository.
• ✅ Change all actions to use a fixed SHA instead of tag/branch reference, and use our forks, while documenting where the source action is located for ease of future update & better understanding.
• Add doc explaining why, to raise devs awareness about this.

Considered

• https://github.com/mheap/pin-github-action
• https://github.com/marketplace/actions/ensure-sha-pinned-actions
• https://github.com/tgymnich/fork-sync

Resources:

• https://michaelheap.com/improve-your-github-actions-security/
• https://michaelheap.com/ensure-github-actions-pinned-sha/