[DOCS] Make the class diagram markers generator-friendly

This is in preparation for adding a script for generating
the class diagram in #1297.

Author: oliverklee

README.md

@@ -619,11 +619,10 @@ class Sabberworm\CSS\CSSList\Document#4 (2) {
 ## Class diagram
 
 ```mermaid
+    %% start of the generated part
 classDiagram
     direction LR
 
-    %% Start of the part originally generated from the PHP code using tasuku43/mermaid-class-diagram
-
     class Anchor {
     }
     class AtRule {
@@ -779,7 +778,6 @@ classDiagram
     ValueList <|-- RuleValueList: inheritance
     Renderable <|.. Comment: realization
     Positionable <|.. Comment: realization
-
     %% end of the generated part