docs: add mixed-workload hierarchical strategy doc and update optimizer READMEs

[Tyndalllll]

Motivation

• Improve documentation discoverability by adding a top-level navigation section to the optimizer READMEs.
• Provide a dedicated design and roadmap document that defines the mixed-workload, hierarchical KV Cache optimization problem and proposed progression.

Description

• Added kv_cache_manager/optimizer/docs/mixed_workload_hierarchical_strategy.md containing a problem definition, taxonomy, and recommended rollout steps for workload-aware, hierarchical KV Cache optimization.
• Updated kv_cache_manager/optimizer/README.md to include a "文档导航" section linking the optimizer overview, analysis scripts, and the new mixed-workload doc.
• Updated kv_cache_manager/optimizer/analysis/script/README.md to include a follow-up section describing how the scripts feed into mixed-workload identification and a link to the new strategy document.

Testing

• This change is documentation-only and did not include automated tests.
• No CI test runs were required for these markdown updates.

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Codex Task