docs: add Doris fine-grained access control guide

[FreeOnePlus]

Summary

• document the MCP control-plane versus Doris data-plane authorization boundary
• add executable table, column, and Row Policy examples with safe rollback steps
• explain which MCP identity routes preserve per-user Doris RBAC
• add English and Simplified Chinese entry points plus a documentation contract test

Validation

• uv run pytest -q -W error (1106 passed, 69 skipped, coverage 58.46%)
• uv run ruff check .
• uv run mypy doris_mcp_server
• uv run bandit -q -c pyproject.toml -r doris_mcp_server
• real Apache Doris 4.0.5: column allow/deny, SELECT * denial, east-only Row Policy, policy inspection, and cleanup all verified with temporary objects

Fixes #82