docs: rewrite README and add CONTRIBUTING.md

[Copilot]

The repo lacked structured documentation — the README had formatting issues, typos, and missing setup context.

README.md

• Added project description lede
• Features list → proper Markdown table
• Fixed typos: "Snapshort", "service principaln"
• Added Prerequisites section and numbered Setup guide with a GitHub Secrets reference table
• Added Customizing KQL Queries section documenting the Title >> KQL line format and a reference table of all 10 built-in checks
• Replaced hardcoded absolute GitHub URLs with relative links

CONTRIBUTING.md (new)

• How to file issues, propose KQL queries, modify the PowerShell script, and submit PRs

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