Add sensory memory documentation

[Shekharrajak]

Linked issue: #350

Purpose of change

This PR adds comprehensive documentation for the Sensory Memory feature in Flink Agents. The documentation covers:

• Overview and key characteristics of Sensory Memory
• When to use Sensory Memory vs other memory types
• Access methods and API usage (Python and Java)
• Practical usage examples
• Auto-cleanup behavior and lifecycle
• Comparison with Short-Term and Long-Term Memory
• Best practices and anti-patterns

The documentation follows the existing documentation style and provides clear guidance for developers using Sensory Memory in their agents.

Tests

Documentation changes do not require code tests. The documentation was verified by:

• Reviewing existing documentation style and format
• Ensuring code examples match the actual API implementation
• Validating that all referenced concepts align with the codebase

API

This change does not touch any public APIs. It is documentation-only.

Documentation

• doc-needed

• doc-not-needed