Add system variable reference

[likidu]

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What is changed, added or deleted? (Required)

Added system variables references doc to list where each variable is referenced in documentation.

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[lilin90]

To make the title grammatically smoother and more standard.

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[likidu]

There are a few things we can improve:

• Hide those variables which don't have any referenced docs. But keep them in the original markdown file for internal references to add them later.
• Resort the order of the referenced files, for example, if there are multiple releases that was referred by a variable, it should be sorted in chronicle order.

[lilin90]

There are a few things we can improve:

• Hide those variables which don't have any referenced docs. But keep them in the original markdown file for internal references to add them later.
• Resort the order of the referenced files, for example, if there are multiple releases that was referred by a variable, it should be sorted in chronicle order.

These two comments have been resolved by several commits above.

Next, I'll handle the custom content for TiDB and TiDB Cloud to make links pass check because some referenced files only apply to one product type.

[lilin90]

Note: I added this line - [System Variables](/system-variables.md) for all variables in this PR via 140d348. So users can easily find the detailed descriptions for each variable.

[ti-chi-bot]

@likidu: 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	7875e9a	link	true	/test pull-verify

Full PR test history. Your PR dashboard.

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