[DOC] Add agentic memory retention policy documentation

[AirFunnyB]

What do you want to do?

• Request a change to existing documentation
• Add new documentation
• Report a technical problem with the documentation
• Other

Tell us about your request. Add documentation for the memory retention policy feature in ML Commons. Covers retention_policy configuration (retention_days, max_count), pinned field, the background retention job, cluster settings, and admin defaults. Draft PR: #12825

Version: 3.8

What other resources are available?

• RFC: [RFC] Memory Retention Lifecycle for Agentic Memory Containers ml-commons#4859
• Implementation PR1 (data model): [Memory Retention] Add retention policy data model, pinned field, and API wiring (1/2) ml-commons#4914
• Implementation PR2 (retention job): [Memory Retention] Add retention job, cluster defaults, and hardening (2/2) ml-commons#4918