docs: Add Lambda Managed Instance mode documentation

https://datadoghq.atlassian.net/browse/SVLS-8083

Overview

Add comprehensive documentation for Lambda Managed Instance support (v90+):

• Overview of Managed Instance mode and how it differs from standard Lambda
• Automatic detection and optimization behavior
• Background continuous flushing architecture with zero per-invocation overhead
• Key differences comparison table (invocation model, flushing, use cases)
• Getting started guide for users

Also clarifies that custom continuous flush intervals are respected in
Managed Instance mode (not completely ignored as previously stated).

Testing

n/a