docs(threatscore): add info with Prowler ThreatScore

[pedrooot]

Description

This PR adds docs about Prowler ThreatScore, how the score is calculated, pillars and sub-pillars and useful user info.

Steps to review

Please add a detailed description of how to review this PR.

Checklist

• Are there new checks included in this PR? Yes / No
  • If so, do we need to update permissions for the provider? Please review this carefully.
• Review if the code is being covered by tests.
• Review if code is being documented following this specification https://github.com/google/styleguide/blob/gh-pages/pyguide.md#38-comments-and-docstrings
• Review if backport is needed.
• Review if is needed to change the Readme.md
• Ensure new entries are added to CHANGELOG.md, if applicable.

API

• Verify if API specs need to be regenerated.
• Check if version updates are required (e.g., specs, Poetry, etc.).
• Ensure new entries are added to CHANGELOG.md, if applicable.

License

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[github-actions]

You can check the documentation for this PR here -> Prowler Documentation

[github-actions]

✅ All necessary CHANGELOG.md files have been updated. Great job! 🎉