docs: rewrite README with accurate summary and proper markdown formatting

[Copilot]

The README lacked a project description, used malformed markdown (no ## headers, no table syntax), and contained typos ("service principaln", "Snapshort").

Changes

• Added project summary — one-paragraph description of what the tool does and its use case
• Fixed markdown — features rendered as a proper | col | col | table; each section promoted to ## heading
• Expanded setup section — secrets documented in a table (AZURE_CLIENT_ID, AZURE_CLIENT_SECRET, AZURE_TENANT_ID, LOG_ANALYTICS_WORKSPACE_ID) with descriptions
• Added How It Works steps — numbered end-to-end flow (auth → query → report → artifact upload)
• Expanded repo structure — includes ms-sentinel-scout-report.html sample report entry
• Fixed typos — "service principaln" → "Azure Service Principal"; "Snapshort" → "Preview"

---

💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.