docs(skill_memory): add Skill Memory documentation for open source

[CaralHsi]

Summary

• New page: content/en/open_source/modules/skill_memory.md — comprehensive documentation for the Skill Memory module
• Covers the full extraction pipeline (task chunking → recall → extract → generate details → package → upload), skill schema fields, SKILL.md output format, extraction modes (simple vs full), configuration (LOCAL/OSS storage), bilingual support, and MemOS integration
• Adds navigation entry in EN settings.yml

Verification

• Cross-referenced against source code: src/memos/mem_reader/read_skill_memory/process_skill_memory.py (1273 lines) and src/memos/templates/skill_mem_prompt.py (534 lines)
• Verified all documented fields match the TreeNodeTextualMemoryMetadata model and create_skill_memory_item() function signatures
• Confirmed extraction pipeline stages match the actual process_skill_memory_fine() implementation

Test plan

• Verify the page renders correctly on the documentation site
• Confirm navigation link works in the sidebar
• Review technical accuracy of pipeline description against latest main branch code

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