Add will-change to excalidraw diagrams.

Adding will-change: filter seems to fix the issue with the Excalidraw diagrams on dark mode and recent Safari versions.
This change was also tested on Firefox and Chrome and it didn't break the diagrams in those browsers.

This fixes #3497.

docs/stylesheets/custom.css

@@ -14,6 +14,7 @@ h3 .doc-heading code {
 }
 
 body[data-md-color-primary="black"] .excalidraw  svg  {
+  will-change: filter;
   filter: invert(100%) hue-rotate(180deg);
 }