one console: replace cluster with instance for Starter and Essential

[qiancai]

…9185)

First-time contributors' checklist

• I've signed the Contributor License Agreement, which is required for the repository owners to accept my contribution.

What is changed, added or deleted? (Required)

Terminology Update: Replaced 'cluster' with 'instance' across documentation for Starter and Essential plans to align with new product terminology.
Navigation Consistency: Updated console navigation paths from 'Clusters' page to 'My TiDB' page for Starter, Essential, and Premium instances, and in some cases, Dedicated clusters.
Improved Clarity in AI/ML Documentation: Refined language in AI/ML-related guides and examples to consistently refer to 'instances' instead of 'clusters' for Starter plans, and clarified connections to 'TiDB Self-Managed' where appropriate.

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 (in Chinese).

• 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:
• Other reference link(s): one console: replace cluster with instance for Starter and Essential docs#22542

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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@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	fee277c	link	true	/test pull-verify

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