functions-and-operators: remove MySQL Reference Manual links (#21821)

[ti-chi-bot]

This is an automated cherry-pick of #21821

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)

• Remove reference manual links for functions for which we have our own docs.
• Keep links for functions/operators that are not explained in the TiDB docs yet
• We can keep a link at the top linking to the corresponding page in the MySQL Reference Manual like this: TiDB supports most of the user-level [locking functions](https://dev.mysql.com/doc/refman/8.0/en/locking-functions.html) available in MySQL 8.0.

This was done with sed -E -i -e 's/(^#* )\[(.*)\].*$/\1\2/' path/to/file.md

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

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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)
• v8.1 (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)?

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• 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

[gemini-code-assist]

Summary of Changes

Hello @ti-chi-bot, 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 streamlines the TiDB documentation by removing redundant external links to the MySQL Reference Manual for functions and operators that are already comprehensively documented within TiDB's own guides. This change aims to provide a more self-contained and consistent user experience, ensuring that users are directed to the most relevant and up-to-date information directly within the TiDB documentation portal. This improves the overall quality and navigability of the documentation.

Highlights

• Documentation Link Cleanup: External MySQL Reference Manual links have been removed from function and operator headings in the TiDB documentation where TiDB provides its own comprehensive documentation.
• Improved User Experience: This change aims to streamline the user experience by directing readers to TiDB's internal documentation for covered topics, reducing reliance on external references.
• Targeted Updates: Only links for functions and operators already explained in TiDB's own documentation were removed, while external links for un-documented items were retained.
• Automated Process: The changes were applied using a sed command, indicating an automated approach to maintain consistency across documentation files.

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[gemini-code-assist]

Code Review

This pull request removes external links to the MySQL Reference Manual from function headings in several documentation files. This change improves readability by decluttering headings and centralizing the reference link at the top of each page. The changes are consistent and correctly implemented according to the pull request's description. The documentation is now cleaner and easier to navigate. I have no further suggestions.

[qiancai]

/approve

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: qiancai

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