Add sysvar reference doc

[likidu]

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

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@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	6c467e9	link	true	/test pull-verify

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