add response function docs

docs/pages/Applications/Optimization_Application/Responses/Linear_Strain_Energy_Response_Function.md

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+---
+title: Linear Strain Energy Response Function
+keywords: 
+tags: [strain energy, response function, optimization]
+sidebar: optimization_application
+summary: 
+---
+## Introduction
+
+This computes the summation of strain energy from each element as the response value and its gradient.
+
+## Formulation
+
+Following formulation is used to compute the summation of strain energy from each element in the chosen model part where $$\underline{u}$$ is the displacement vector for the element and $$\mathbf{K}$$ is the stiffness matrix of the element.
+<p align="center">$$ J   = \frac{1}{2}\sum_{\forall \Omega_i \in \Omega} \underline{u}^T_i \mathbf{K}_i \underline{u}_i  $$</p>
+
+## Json settings
+Following code snippet illustrates json settings used in this response function.
+```json
+{
+    "name": "strain_energy",
+    "type": "linear_strain_energy_response_function",
+    "settings": {
+        "evaluated_model_part_names": [
+            "Structure"
+        ],
+        "primal_analysis_name": "Structure_static",
+        "perturbation_size": 1e-8
+    }
+}
+```
+
+| Option | Allowed values |
+| ------------- | ------------- |
+| name | A unique string |
+| type  | "linear_strain_energy_response_function"  |
+| evaluated_model_part_names  | List of model part names to compute strain energy |
+| primal_analysis_name | Name of the analysis from the list of analysis to be used for strain energy computation |
+| perturbation_size | Perturbation size to be used in the semi-analytic method |
+
+## Source files
+
+* [applications/OptimizationApplication/python_scripts/responses/linear_strain_energy_response_function.py](https://github.com/KratosMultiphysics/Kratos/blob/master/applications/OptimizationApplication/python_scripts/responses/linear_strain_energy_response_function.py)
+* [Doxygen](doxygen) TODO

docs/pages/Applications/Optimization_Application/Responses/Mass_Response_Function.md

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+---
+title: Mass Response Function
+keywords: 
+tags: [mass, response function, optimization]
+sidebar: optimization_application
+summary: 
+---
+
+## Introduction
+
+This response function computes the mass of the given list of model parts.
+
+## Formulation
+
+Following formulation is used.
+<p align="center">$$ J   = \sum_{\forall \Omega_i \in \Omega} m_i $$</p>
+
+## Json settings
+Following code snippet illustrates json settings used in this response function.
+```json
+{
+    "name": "mass",
+    "type": "mass_response_function",
+    "settings": {
+        "evaluated_model_part_names": [
+            "Structure"
+        ]
+    }
+}
+```
+
+| Option | Allowed values |
+| ------------- | ------------- |
+| name | A unique string |
+| type  | "mass_response_function"  |
+| evaluated_model_part_names  | List of model part names to compute mass |
+
+## Source files
+
+* [applications/OptimizationApplication/python_scripts/responses/mass_response_function.py](https://github.com/KratosMultiphysics/Kratos/blob/master/applications/OptimizationApplication/python_scripts/responses/mass_response_function.py)
+* [Doxygen](doxygen) TODO

docs/pages/Applications/Optimization_Application/Responses/menu_info.json

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+        "Overview.md"
+    ]
+}