[doc](lakehouse) Add external metadata cache memory management for 4.1.5

[CalvinKirs]

What

• Add Chinese and English usage guides for external metadata cache memory management, supported from Doris 4.1.5.
• Document FE-, Catalog-, and cache-entry-level memory limits and their configuration hierarchy.
• Cover the supported Hive, Iceberg, and Paimon cache entries, configuration examples, eviction behavior, and operational guidance.
• Document the final accounting model: type-specific conservative formulas plus loader-time payload counters, with no private-field reflection or container sampling.
• Explain Iceberg's non-growing cached metadata generation and statement-local reads from the key-pinned metadataFileLocation.
• Explain soft-value collection, reservation records without strong value ownership, O(1) prepared-weight lookup, and the post-load admission/OOM boundary.
• Add the guide to the 4.x sidebar and link it from the metadata cache page.
• Clarify that Hive cache properties are reloaded from Doris 4.1.5 by rebuilding the affected Catalog cache.

Why

This documents the user-facing configuration and runtime behavior introduced by apache/doris#66717, especially the cases discussed during review:

• A Catalog or cache-entry limit can be configured without an FE-wide limit.
• Oversized or safely unmeasurable objects are returned to the query but are not cached; cache admission does not fail the query.
• The quota is checked after object construction and therefore is not pre-load OOM protection.
• Shared infrastructure such as Catalog, FileIO, and thread pools is excluded from a single cache entry's ownership.
• Weight-bounded values may be collected under heap pressure without a reservation record keeping the value alive.
• Iceberg statements remain bound to one metadata generation during concurrent cache refresh.
• Only explicitly supported cache entries accept max-weight; unsupported keys are rejected instead of being silently ignored.

Checks

• git diff --check
• Parsed both Chinese and English 4.x document frontmatters as JSON.
• Verified balanced and matching Markdown code fences in both documents.
• Verified the existing single 4.x sidebar entry remains unchanged.

A full Docusaurus build was not run because dependencies are not installed in the clean worktree.