release-8.5: integrate Starter and Essential to cloud docs

[lilin90]

What is changed, added or deleted? (Required)

• Merged Starter and Essential docs to docs.pingcap.com
• Updated content caused by renaming Serverless to Starter

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[gemini-code-assist]

Summary of Changes

Hello @lilin90, 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!

I've significantly updated the TiDB Cloud documentation to integrate the newly introduced 'Essential' tier and to reflect the renaming of the 'Serverless' tier to 'Starter'. This change aims to provide clearer guidance for users selecting and managing their TiDB Cloud clusters, from free-tier exploration to production-grade deployments. The updates span across core concepts, development guides, operational procedures, and CLI references, ensuring that all relevant information is consistent with the new tier structure and terminology. This consolidation and clarification will enhance the user experience by streamlining access to pertinent documentation for each TiDB Cloud offering.

Highlights

• New 'Essential' Tier: I've introduced a new 'Essential' tier for TiDB Cloud, designed for production-ready workloads with provisioned capacity and automatic scaling. This tier is currently in public preview on Alibaba Cloud.
• Renaming of 'Serverless' to 'Starter': The 'Serverless' tier has been officially renamed to 'Starter' to better reflect its purpose as an entry point for building with TiDB Cloud. All documentation has been updated to reflect this new terminology.
• Dedicated Documentation TOCs: I've created dedicated Table of Contents (TOC) files for both the 'Starter' and 'Essential' tiers, providing a clearer and more organized documentation structure for users of these specific plans.
• Extensive Documentation Updates: Numerous documentation files have been updated to reflect the introduction of the 'Essential' tier and the 'Starter' renaming. This includes updates to compatibility notes, feature availability, connection instructions, and CLI command descriptions across various guides and reference materials.
• New CLI Command for Essential Cluster Capacity: The ticloud serverless capacity CLI command has been introduced to allow users to set minimum and maximum Request Capacity Units (RCUs) for 'Essential' clusters, providing programmatic control over their scaling behavior.

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[gemini-code-assist]

Code Review

This pull request effectively integrates the 'Starter' and 'Essential' tier documentation into the main cloud docs, including renaming 'Serverless' to 'Starter'. The changes are extensive and mostly look good, with new Table of Contents files for the different tiers and updates across many documents to reflect the new structure. I've found a bug in a new script and several areas in the documentation where clarity and consistency can be improved. My comments include a fix for the script and suggestions to enhance readability and accuracy in the documentation.