docs(mxgraph): update references to custom code (#3137)

Add links to the source code.
Fix reference to files (ShapeConfigurator is no longer in charge of
overriding SvgCanvas2D).

docs/contributors/mxgraph-integration.md

@@ -29,7 +29,7 @@ The default mxGraph parent is the parent of
 
 The bpmn input source coordinates are absolute whereas mxgraph uses coordinates in the referential of the parent cell.
 As we define a parent-child relationship when inserting BPMN elements in the mxGraph model, a coordinate transformation
-layer is required. See `BpmnRenderer` for more details.
+layer is required. See [BpmnRenderer](../../src/component/mxgraph/BpmnRenderer.ts) for more details.
 
 
 ## BPMN Elements rendering and style
@@ -44,7 +44,7 @@ The [style](https://jgraph.github.io/mxgraph/docs/js-api/files/view/mxStylesheet
 is defined and registered in `StyleConfigurator`. In particular, it refers to the name of a [mxShape](https://jgraph.github.io/mxgraph/docs/js-api/files/shape/mxShape-js.html)
 used for the rendering.
 
-The `mxShape` can be a standard `mxGraph` class or a custom BPMN `mxShape` defined by the `bpmn-visualization`. The custom `mxShapes` are registered by `ShapeConfigurator`
+The `mxShape` can be a standard `mxGraph` class or a custom BPMN `mxShape` defined by the `bpmn-visualization`. The custom `mxShapes` are registered by [ShapeConfigurator](../../src/component/mxgraph/config/ShapeConfigurator.ts).
 which associates the `mxShape` name (used in style definition) with the `mxShape` class to be used.
 
 For more details, see [BPMN Support - How To](./bpmn-support-how-to.md).
@@ -142,16 +142,16 @@ to see various positioning methods in action.
 We are hacking mxCellOverlays which originally only supports image shape. This lets us use custom shapes.
 
 Customization main points
-- `MxGraphCustomOverlay`: add extra configuration and behavior to mxCellOverlay
-- customized `mxCellRenderer`: change the overlays rendering code to transform `MxGraphCustomOverlay` into specific shapes 
+- [MxGraphCustomOverlay](../../src/component/mxgraph/overlay/custom-overlay.ts): add extra configuration and behavior to `mxCellOverlay`.
+- [BpmnCellRenderer](../../src/component/mxgraph/BpmnCellRenderer.ts) (customized `mxCellRenderer`): change the overlays rendering code to transform `MxGraphCustomOverlay` into specific shapes 
 according to its configuration.
 
 More details are available in [#955](https://github.com/process-analytics/bpmn-visualization-js/issues/955).
 
 
-## Custom CSS classes added to svg node generated by mxGraph
+## Custom CSS classes added to SVG node generated by mxGraph
 
-In `ShapeConfigurator`, we customize the mxShape rendering code to add extra CSS classes depending on BPMN Element kinds.
-It allows styling to be managed with external CSS classes and it is also used for element identification and selection.
+In [BpmnCellRenderer](../../src/component/mxgraph/BpmnCellRenderer.ts), we customize the `mxShape` rendering code to add extra CSS classes depending on BPMN Element kinds.
+It allows styling to be managed with external CSS classes, and it is also used for element identification and selection.
 
 More details are available in [#942](https://github.com/process-analytics/bpmn-visualization-js/issues/942).