add interface help page, re #12610

Code/Mantid/docs/source/interfaces/Engineering_Diffraction.rst

+Tomographic Reconstruction
+==========================
+
+.. contents:: Table of Contents
+  :local:
+
+Overview
+--------
+
+This custom interface integrates several taks related to engineeering
+diffraction. This is under heavy development, at the moment the
+interface only provides calibration related functionality. The
+following sections will describe the different sections or tabs of the
+interface.
+
+.. interface:: Engineering Diffraction
+  :align: center
+  :width: 400
+
+Calibration
+-----------
+
+This tab provides a graphical interface to calculate calibrations and
+visualize them.
+
+.. interface:: Engineering Diffraction
+  :widget: wCalib
+  :align: right
+  :width: 300
+
+Settings
+--------
+
+Controls several settings, including the input folders where the
+intrument run files can be found.
+
+.. interface:: Engineering Diffraction
+  :widget: wSettings
+  :align: right
+  :width: 300
+
+.. categories:: Interfaces Diffraction