Re #17144 Initial documentation work

docs/source/interfaces/Engineering_Diffraction.rst

+.. _Engineering_Diffraction-ref:
+
 Engineering Diffraction
 =======================
 

docs/source/interfaces/Engineering_Diffraction_TestGuide.rst

+.. _Engineering_Diffraction_TestGuide-ref:
+
+Engineering Diffraction Testing
+=================================
+
+.. contents:: Table of Contents
+    :local:
+    
+Preamble
+^^^^^^^^^
+This document is tailored towards developers intending to test the Engineering Diffraction
+interface. Whilst users may benefit from the extended examples provided in this document
+it is recommended they read :ref:`Engineering_Diffraction-ref:` which is tailored towards users.
+
+The files used in the following examples are found within System test files within 
+`<MantidBuildDir>/ExternalData/Testing/Data/SystemTest` after building the *SystemTestData* target.
+
+Overview
+^^^^^^^^
+The Engineering Diffraction interface allows scientists using the EnginX instrument to interactively
+process their data. There are 5 tabs from which are ordered according to the main steps performed.
+These are:
+
+- Calibration - This is where a vanadium and cerium run are entered to calibrate the subsequent data.
+- Focus - This is where the data can be masked to only use specific detectors or all.
+- Pre-processing - Event workspaces have to be converted into histogram data before 
+  focusing which can be completed here.
+- Fitting - The user uses their focused data here to interactively pick peaks for subsequent fitting.
+- Settings - This holds additional settings which may be of use during testing.
+
+Calibration
+^^^^^^^^^^^
+The first time the interface is opened it will be disabled except for the RB number entry box
+and a pop up box which should prompt for a RB number. For testing purposes this can be anything.
+
+If a vanadium calibration has not previously been run the `Current calibration` will be empty
+and the following files need selected and calibration run.
+- *Vanadium#* ENGINX00236516.nxs
+- *Calibration#* ENGINX00194547.nxs
+
+If `Current calibration` is populated the algorithm will attempt to use a cached calculation instead
+of recalculating the calibration. To force it to recalculate every time for testing purposes the
+option can be set under :ref:`settings-Engineering_Diffraction-ref:`.
+
+
+
+
+.. _settings-Engineering_Diffraction_TestGuide-ref
+
+Settings
+^^^^^^^^^
\ No newline at end of file