Update CREATE INDEX statement docs

[qiancai]

What is changed, added or deleted? (Required)

• Uppercase SQL keywords
• Remove copyable hints
• Add more links

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.

• master (the latest development version)
• v8.1 (TiDB 8.1 versions)
• v8.0 (TiDB 8.0 versions)
• v7.6 (TiDB 7.6 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)
• v5.3 (TiDB 5.3 versions)
• v5.2 (TiDB 5.2 versions)
• v5.1 (TiDB 5.1 versions)

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

• This PR is translated from: Update CREATE INDEX statement docs docs#17220
• 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

[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-05-15 08:11:30.053882283 +0000 UTC m=+1640843.811017857: ☑️ agreed by hfxsd.

[ti-chi-bot]

@hfxsd: Your lgtm message is repeated, so it is ignored.

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[qiancai]

/approve

[ti-chi-bot]

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[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.0: #17352.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.1: #17353.