docs: add excalidraw codebase section

dev-docs/docs/codebase/introduction-to-the-codebase.mdx

@@ -0,0 +1,6 @@
+---
+title: Introduction to the codebase
+slug: ../
+---
+
+This section is documenting the Excalidraw codebase itself for developers who want to contribute to the project.

dev-docs/sidebars.js

@@ -92,6 +92,16 @@ const sidebars = {
         "@excalidraw/excalidraw/development",
       ],
     },
+
+    {
+      type: "category",
+      label: "Excalidraw codebase",
+      link: {
+        type: "doc",
+        id: "codebase/introduction-to-the-codebase",
+      },
+      items: [],
+    },
   ],
 };