docs(open_source_api): add EN translation of all open-source REST API docs

[CaralHsi]

Summary

• 15 new pages: Full English translation of the open-source REST API documentation section, previously available only in Chinese
• Categories covered:
  • Start (2 files): Overview, Configuration
  • Core (5 files): Add Memory, Search Memory, Delete Memory, Get Memories, Get Memory Detail
  • Chat (1 file): Chat (complete + stream)
  • Message (3 files): Feedback, Get Message, Get Suggestions
  • Scheduler (1 file): Task Status & Queue Monitoring
  • Tools (2 files): Check Cube Existence, Get User Names
  • Help (1 file): Error Codes
• Adds full navigation section in EN settings.yml

Verification

• All endpoint paths verified against server_router.py route definitions (e.g., POST /product/add, POST /product/search, GET /product/get_memory/{memory_id}, GET /product/scheduler/status)
• Parameter tables cross-checked with product_models.py Pydantic model fields (APIADDRequest, APISearchRequest, ChatRequest, APIFeedbackRequest, DeleteMemoryRequest, etc.)
• Handler orchestration descriptions verified against actual handler classes (AddHandler, SearchHandler, ChatHandler, FeedbackHandler)
• SDK code examples verified against MemOSClient method signatures

Test plan

• Verify all 15 pages render correctly on the documentation site
• Confirm all navigation links resolve properly in the EN sidebar
• Spot-check 2-3 parameter tables against latest main branch Pydantic models
• Verify internal cross-links between pages (e.g., Add Memory → Search Memory)

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