docs(getting-started): add information about slsa attestor

[rrey]

What this PR does / why we need it

This PR results in a short discussion on Slack.

The Provenance being the very first requirement from SLSA, it seems quite appropriate to mention how Witness helps achieve this easily.

Acceptance Criteria Met

• [ ] Docs changes if needed
• [ ] Testing changes if needed
• [ ] All workflow checks passing (automatically enforced)
• [ ] All review conversations resolved (automatically enforced)
• [ ] DCO Sign-off

Special notes for your reviewer:

[netlify[bot]]

✅ Deploy Preview for witness-project ready!

Name	Link
🔨 Latest commit	5c1c1f10c89d516e3ebf03655bfe6b75593ab3a8
🔍 Latest deploy log	https://app.netlify.com/sites/witness-project/deploys/665aa8c6f46a7a00080717d0
😎 Deploy Preview	https://deploy-preview-456--witness-project.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify site configuration.

[rrey]

Changing to draft to see if I find other issues while testing witness. I'll remove the draft flag at the end of the day if I do not find anything else.