added a doc file explaining slicing call hierarchy (#7796)

doc/developer-reference/Home.md

@@ -4,3 +4,4 @@ This is a documentation from someone exploring the code and is by no means compl
 
 - [Preset, PresetBundle and PresetCollection](https://github.com/SoftFever/OrcaSlicer/blob/main/doc/developer-reference/Preset-and-bundle.md)
 - [Plater, Sidebar, Tab, ComboBox](https://github.com/SoftFever/OrcaSlicer/blob/main/doc/developer-reference/plater-sidebar-tab-combobox.md)
+- [Slicing Call Hierarchy](https://github.com/SoftFever/OrcaSlicer/blob/main/doc/developer-reference/slicing-hierarchy.md)

doc/developer-reference/slicing-hierarchy.md

@@ -0,0 +1,5 @@
+## Slicing Call Hierarchy
+
+The Slicing logic is not the easiest to locate in the code base. Below is a flow diagram of function calls that are made after clicking the `Slice Plate` button in the UI. Most of the processing happens in different threads. Note the calls after `BackgroundSlicingProcess::start()`, but this is how you can find the slicing logic.
+
+<img src="../images/slicing_call_heirarchy.svg" alt="Example Image" width="320">