[Positioning] Add 'Use this when / Use something else when' section

Goal

Set expectations explicitly; reduce wrong-fit issues.

What to ship

A README section near the top:

Use this when…

• You're building an AI agent or LLM-powered app that needs to read or modify data in Azure DocumentDB.
• You need Entra-secured access from a hosted MCP endpoint.
• You want defense-in-depth (RBAC, allow/deny lists, audit logging) for an LLM-facing data surface.

Use something else when…

• You're targeting MongoDB Atlas → see mongodb-mcp-server.
• You're writing CRUD code by hand → use the DocumentDB SDK directly.
• You're doing routine DB admin → use Compass, mongosh, or the Azure portal.
• You need a query result set with millions of rows → MCP is not the right shape; use the SDK.

Acceptance criteria

• Clear pointers to alternatives in the "not us" bullets.
• Reviewed for accuracy (don't disparage alternatives — just point to them).

Depends on

• Positioning Decision: Audience.