[docs] Improve POC guide with bucket sizing rules and heading cleanup

[dataroaring]

Summary

• Replace bucket count guidance with clear four-rule approach (BE multiple, minimize count, 20GB/10GB cap, 128 max per partition)
• Rename section headings for clarity: Key Columns → Sort Key, Typical Use Cases → Example Templates, Common Performance Pitfalls → Performance Pitfalls
• Merge sparse partition section into a single paragraph
• Remove unnecessary "Fixing Mistakes" section
• Fix broken CDC sync link and rule count reference

Test plan

• Verify docs build successfully
• Check EN and ZH POC guide pages render correctly
• Confirm all internal links resolve

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Improves the POC table-design guide by clarifying bucket sizing guidance (four-rule approach), cleaning up headings, and simplifying/streamlining sections in both EN and ZH docs.

Changes:

• Replaces prior bucket-count guidance with a clearer four-rule approach and updates related references.
• Renames several headings for clarity and consistency (e.g., Key Columns → Sort Key).
• Consolidates/simplifies partitioning guidance and removes the “choose wrong” remediation section.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
i18n/zh-CN/docusaurus-plugin-content-docs/current/gettingStarted/must-read-before-poc.md	Updates ZH headings/wording to match the new bucket rules and streamlined layout.
docs/gettingStarted/must-read-before-poc.md	Updates EN headings/wording to match the new bucket rules and streamlined layout.

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[Copilot]

The section heading was renamed to “Sort Key”, but this paragraph still refers to “Key columns” throughout. To avoid confusing readers, update the terminology to consistently use “Sort Key” (or “sort key columns”) in this explanation (including “first 36 bytes of …”).

Suggested change

### 2. Sort Key

**Why it matters:** The sort key determines the **physical sort order** on disk. Doris builds a [prefix index](../table-design/index/prefix-index) on the first 36 bytes of the sort key columns, so queries that filter on these columns run significantly faster. However, when a `VARCHAR` column is encountered, the prefix index stops immediately — no subsequent columns are included. So place fixed-size columns (INT, BIGINT, DATE) before VARCHAR to maximize index coverage.

[Copilot]

In the ZH doc, most headings are primarily Chinese, but this one is English-first (“Sort Key（排序键）”). For consistency/readability in the localized page, consider switching to a Chinese-first form like “排序键（Sort Key）” (or fully Chinese if that matches the rest of the page style).

Suggested change

	POC 阶段，**Duplicate Key 适用于大多数场景**。只有在明确需要更新或预聚合时才切换。详细对比见[数据模型概述](../table-design/data-model/overview)。
	## 2. 排序键（Sort Key）