Update dm worker intro

[alastori]

What is changed, added or deleted? (Required)

This PR updates the dm-worker-intro.md document to improve its clarity, accuracy, and completeness.

• Replaced "Features" with "Key Concepts": The previous "Features" list was inaccurate and confusing. It has been replaced with a "Key Concepts" section that correctly describes the worker's function, including its handling of full and incremental data, its high-availability capabilities, and the "one worker to one source" rule.
• Added a new Note block: A prominent note was added to clarify that DM-worker is a binlog client, not a standby replica, and to direct users to TiCDC for migrating data from a TiDB cluster.
• Expanded "Upstream Privileges" section: The original privilege list was insufficient for newer MariaDB versions. The documentation now provides separate, accurate privilege lists and GRANT examples for:
  • MySQL and MariaDB (< 10.5)
  • MariaDB (10.5 and later)
• Added workaround for MariaDB: A new note explains why the check-task command might fail with MariaDB 10.5+ and provides the ignore-checking-items workaround.

Which TiDB version(s) do your changes apply to? (Required)

Tips for choosing the affected version(s):

By default, CHOOSE MASTER ONLY so your changes will be applied to the next TiDB major or minor releases. If your PR involves a product feature behavior change or a compatibility change, CHOOSE THE AFFECTED RELEASE BRANCH(ES) AND MASTER.

For details, see [tips for choosing the affected versions](https://github.com/pingcap/docs/blob/master/CONTRIBUTING.md#guideline-for-choosing-the-affected-versions).

• master (the latest development version)
• v9.0 (TiDB 9.0 versions)
• v8.5 (TiDB 8.5 versions)
• v8.1 (TiDB 8.1 versions)
• v7.5 (TiDB 7.5 versions)
• v7.1 (TiDB 7.1 versions)
• v6.5 (TiDB 6.5 versions)
• v6.1 (TiDB 6.1 versions)
• v5.4 (TiDB 5.4 versions)

What is the related PR or file link(s)?

• This PR is translated from: N/A
• Other reference link(s):
  • DM fails when checking privileges due to unsupported syntax on MariaDB tiflow#12207
  • File changed: /dm/dm-worker-intro.md

Do your changes match any of the following descriptions?

• Delete files
• Change aliases
• Need modification after applied to another branch
• Might cause conflicts after applied to another branch

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[gemini-code-assist]

Summary of Changes

Hello @alastori, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request significantly enhances the dm-worker-intro.md documentation by improving its accuracy, clarity, and completeness. It refines the description of DM-worker's core functionalities, clarifies its relationship to other tools like TiCDC, and provides crucial, version-specific updates to upstream database privilege requirements, including a practical workaround for MariaDB 10.5+ compatibility issues.

Highlights

• Documentation Clarity & Accuracy: The introduction to DM-worker has been significantly revised. The vague 'Features' list was replaced with a more precise 'Key Concepts' section that details its operational role, including full and incremental replication, high-availability aspects, and the 'one worker to one source' rule. This improves the understanding of DM-worker's core functionalities.
• Role Clarification Note: A prominent note was added to explicitly state that DM-worker is a MySQL binlog client, not a standby replica. This note also guides users to TiCDC for replicating data from a TiDB cluster, preventing common misunderstandings.
• MariaDB 10.5+ Privilege Updates: The upstream database privilege requirements have been expanded and made version-specific. The documentation now provides distinct and accurate GRANT examples for MySQL/MariaDB (< 10.5) and MariaDB (10.5 and later), addressing changes in MariaDB's privilege system and ensuring correct setup for newer versions.
• check-task Workaround for MariaDB: A new note and workaround have been added to address potential check-task failures with MariaDB 10.5+. This provides practical guidance on using the ignore-checking-items option in task configurations when encountering privilege errors, improving user experience.

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[alastori]

cc @OliverS929

[gemini-code-assist]

Code Review

This is a great update that significantly improves the accuracy and clarity of the DM-worker documentation. The restructuring of the privileges section to account for different MariaDB versions is particularly helpful. I've added a few minor suggestions to enhance readability and correct some markdown formatting. Overall, excellent work!