cloud release-8.1: remove serverless specific docs

[qiancai]

What is changed, added or deleted? (Required)

This PR removes TiDB Cloud Serverless specific docs from TiDB Cloud Documentation at https://docs.pingcap.com/tidbcloud/.

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

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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.

• master (the latest development version)
• v9.0 (TiDB 9.0 versions)
• v8.5 (TiDB 8.5 versions)
• v8.4 (TiDB 8.4 versions)
• v8.3 (TiDB 8.3 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)

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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]

Summary of Changes

Hello @qiancai, 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 primarily focuses on streamlining and updating the documentation for the release-8.1 branch. It involves removing outdated or serverless-specific content, reorganizing existing sections for better clarity, and adding new documentation for features and tools relevant to TiDB's self-managed and dedicated cloud offerings. The changes aim to provide a more accurate, comprehensive, and user-friendly documentation experience for the current release.

Highlights

• Documentation Scope Adjustment: The pull request significantly refines the documentation scope for the release-8.1 branch by removing content specifically tied to TiDB Cloud Serverless, as indicated by the PR title and changes in TOC-tidb-cloud.md and _docHome.md.
• Content Reorganization and Expansion: Key documentation files like TOC-tidb-cloud.md and TOC.md have undergone extensive restructuring. This includes the introduction of new sections for 'Key Concepts', 'Vector Search', and 'Disaster Recovery', alongside detailed API and CLI references, indicating a shift towards a more structured and comprehensive guide for dedicated/self-managed offerings.
• Version Alignment and Maintenance: The README.md and TOC.md files are updated to reflect release-8.1 as a Long-Term Support (LTS) version, and new release notes for various versions (v8.5, v8.1, v7.5, v7.1, v6.5) have been added, ensuring the documentation remains current and accurate.
• Terminology Standardization: Across numerous documentation files, terminology has been standardized, with 'TiDB Serverless' updated to 'TiDB Cloud Serverless', 'TiDB Dedicated' to 'TiDB Cloud Dedicated', and 'TiDB Self-Hosted' to 'TiDB Self-Managed', improving consistency and clarity.
• New Language Support and Utilities: A new C# sample application guide (develop/dev-guide-sample-application-cs.md) has been added, and new utility functions (functions-and-operators/utility-functions.md) and TiDB-specific functions (functions-and-operators/tidb-functions.md) are now documented, expanding the range of supported development practices.

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[gemini-code-assist]

Code Review

This pull request removes entries for TiDB Cloud Serverless from the main Table of Contents (TOC-tidb-cloud.md), which aligns with the stated goal of removing serverless-specific documentation.

[ti-chi-bot]

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