docs: add write-update-tidb-docs skill

[qiancai]

What is changed, added or deleted? (Required)

This PR adds the .ai/skills/write-update-tidb-docs/ skill and registers it in AGENTS.md and .ai/AI-README.md.

The skill helps agents turn engineering and product context into user-facing TiDB documentation. It covers:

• Accepted inputs such as code PRs, GitHub issues, product specs, design docs, rough notes, and existing docs.
• Target-branch and documentation-impact analysis.
• Separate workflows for creating a new documentation page and updating existing pages.
• Validation and completion-report expectations.

Benefits:

• Gives PM and R&D contributors a repeatable way to request docs work from implementation context.
• Keeps AI-assisted edits aligned with repo conventions, writing style, templates, TOC placement, and validation.
• Reduces missed related-doc impacts such as system variables, config parameters, SQL statements, and compatibility notes.
• Keeps new-page creation and existing-page updates narrowly scoped and reviewable.

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

• 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)

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

• This PR is translated from:
• Other reference link(s):

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

[gemini-code-assist]

Code Review

This pull request introduces a new AI skill write-update-tidb-docs to assist in writing and updating TiDB documentation. It adds the main skill definition (SKILL.md), reference workflows for creating new documents (ref-create-new-doc.md) and updating existing ones (ref-update-existing-doc.md), and registers the skill in AI-README.md and AGENTS.md. The reviewer's feedback consistently points out violations of the repository style guide regarding heading capitalization, recommending that all headings, placeholders, and title guidelines use sentence case instead of title case.

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@qiancai: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-verify	3530080	link	true	/test pull-verify

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