Add the missing documents for two-phase flow with PP formulation

web/content/docs/benchmarks/two-phase-flow-PP/TwoPhaseFlowPP.md

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+project = "Parabolic/TwoPhaseFlowPP"
+author = "Yonghui Huang"
+date = "2018-06-20T14:37:58+01:00"
+title = "Isothermal two-phase flow transport model with PP formulation"
+weight = 101
+
+[menu]
+  [menu.benchmarks]
+    parent = "two-phase-flow-PP-formulation"
+
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+
+{{< data-link >}}
+
+## Problem description
+
+Two benchmarks are presented here which are dedicated to simulating the two-phase flow transport in porous media with the pessure-capillary pressure as the primary variables. For the detailed benchmark settings and results description, please refer to the attached document.
+
+## Results and evaluation
+
+See [this PDF](../main.pdf).