Auto-generate sysvar documentation from TiDB server source

[morgo]

What is changed, added or deleted? (Required)

This auto-generates the system variables documentation by using the TiDB server's source code. The advantage of doing this is that system-variables will be guaranteed accurate, and non-documented sysvars will be automatically discovered.

It has a few small differences in formatting, but I've tried to keep it as close to the current document as makes sense.

** This contributes the script for future use by others. Generated system-variables.md files will be contributed separately. **

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

• master (the latest development version)
• v5.1 (TiDB 5.1 versions)
• v5.0 (TiDB 5.0 versions)
• v4.0 (TiDB 4.0 versions)
• v3.1 (TiDB 3.1 versions)
• v3.0 (TiDB 3.0 versions)
• v2.1 (TiDB 2.1 versions)

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

• This PR is translated from:
• 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]

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• dveeden

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[TomShawn]

@morgo This is just awesome 😱! I guess this PR also applies to release-5.0, because the most majority of the content in release-5.0 is the same with that in master.

[morgo]

@morgo This is just awesome ! I guess this PR also applies to release-5.0, because the most majority of the content in release-5.0 is the same with that in master.

That's actually complicated. The TiDB source code in release-5.0 does not support the required features to auto-generate. Only master does. I have also been fixing bugs based on the possible range of numeric types, so I think it's better to draw a line in the sand and say it only applies to docs in 5.1+

[qiancai]

Removed the needs-cherry-pick-release-6.6 label because the v6.6 docs have been archived at https://docs-archive.pingcap.com/tidb/v6.6 and will no longer receive new updates.