Add long-term memory documentation

[Shekharrajak]

Linked issue: #351

Purpose of change

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

• Overview and key characteristics of Long-Term Memory
• Configuration and setup requirements
• Memory set operations (create, add, get, search, delete)
• Compaction strategies (summarization) and async compaction
• Practical usage examples (conversation history, knowledge bases, RAG applications)
• Comparison with other memory types (Sensory Memory, Short-Term Memory)

Tests

• Ensuring code examples match the actual API implementation in python/flink_agents/api/memory/long_term_memory.py
• Validating that all referenced concepts align with the codebase
• Referencing the e2e test file long_term_memory_test.py for usage patterns

API

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

Documentation

• doc-needed

• doc-not-needed