Add system variable reference

[likidu]

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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.