[Docs] Refine and restructure the Unified External Meta Cache guide

[suxiaogang223]

Related PRs

This PR updates the documentation related to the new unified external metadata cache framework introduced in:

• [Refactor](Multi Catalog) Unify external meta cache framework doris#60937: [Refactor](Multi Catalog) External meta cache framework with engine adapters

What this PR does / why we need it

This PR refines the user guide for the unified external metadata cache (unified-meta-cache.md in both English and Chinese) to improve its readability, structural flow, and accuracy.

Key improvements include:

1. Structural Reorganization: Moved the "What External Meta Cache Includes" section before the configuration section to help users build a clear mental model of the cache architecture (Catalog vs. Engine level) before applying configurations.
2. Table Consolidation: Merged the redundant "Support Matrix" and "Catalog-Specific Configuration" tables into a single, high-density reference table to improve lookup efficiency.
3. Clarified Semantics:
   • Added explicit notes on the logical conditions required for a cache to take effect (e.g., enable=true AND ttl-second > 0 AND capacity > 0).
   • Standardized terminology across the document by renaming <module> to <entry>, strictly aligning with the ENTRY_NAME column in the catalog_meta_cache_statistics system table.
4. Enhanced Observability Guide: Added detailed descriptions for critical troubleshooting metrics like LOAD_FAILURE_COUNT and LAST_ERROR to help users diagnose upstream metadata service issues.
5. Emphasized Limitations: Added explicit :::caution blocks to highlight that Hive meta.cache.hive.* properties do not currently support hot-reload via ALTER CATALOG.

Check List

• English documentation updated
• Chinese documentation updated

[suxiaogang223]

run buildall