docs: add key management provider and cosign docs

[akashsinghal]

This PR builds on #69 and MUST be merged after.

Adds new documentation for Key Management Providers:

• adds new Key Management Provider resource docs under CRDs section
• general docs clean up and title renaming
• Moves some docs around
• adds error troubleshooting documentation

Adds new documentation for Cosign verifier

• adds new demo and walkthrough
• adds section on trust policies

fixes #47

[netlify[bot]]

✅ Deploy Preview for ratify-dev ready!

Name	Link
🔨 Latest commit	50ae74fd13b9c8de3bd25680a599c47c0d267a7a
🔍 Latest deploy log	https://app.netlify.com/sites/ratify-dev/deploys/66467c3d4d38270008551d04
😎 Deploy Preview	https://deploy-preview-58--ratify-dev.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.