feat: add .env.markflow configuration and update wiki workflow to use it

Author: darmie

book/.env .env.markflowbook/.env renamed to .env.markflow

@@ -94,7 +94,7 @@ jobs:
 
           # Build book docs with Markflow into /book subdirectory
           markflow build --prebake --source ./book \
-            --output ./site/book --config ./book/.env
+            --output ./site/book --config .env.markflow
 
       - name: Configure Pages
         uses: actions/configure-pages@v5