sql: make global indexes a new doc and add more contents (#20855)

[ti-chi-bot]

This is an automated cherry-pick of #20855

What is changed, added or deleted? (Required)

Introduced a new 'global-indexes.md' document detailing global indexes, their use cases, limitations, and implementation. Updated references and links across multiple files to point to the new documentation, replacing inlined or outdated content. Removed the inlined global index section from 'partitioned-table.md' and adjusted related documentation and best practices to reference the new page. Added supporting images for global index mechanisms and comparisons.

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

• This PR is translated from:
• Other reference link(s):https://tidb.net/blog/7077577f#TiDB%20%E5%85%A8%E5%B1%80%E7%B4%A2%E5%BC%95%E7%9A%84%E4%BC%98%E5%8A%BF

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

[ti-chi-bot]

@hfxsd This PR has conflicts, I have hold it.
Please resolve them or ask others to resolve them, then comment /unhold to remove the hold label.

[ti-chi-bot]

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[lilin90]

/approve

[ti-chi-bot]

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