ci: Build mdbook from main branch

It was yet another hardcoded version we had to bump before release.
If book in main gets too different means we need a release anyway.

.github/workflows/mdbook.yml

@@ -2,7 +2,7 @@ name: Deploy mdBook site to Pages
 
 on:
   push:
-    branches: ["2.1"]
+    branches: ["main"]
 
   # Allows you to run this workflow manually from the Actions tab
   workflow_dispatch: