📝 Refactor docs, "Tutorial - User Guide" and "Advanced User Guide" (#887)

docs/css/custom.css

@@ -1,3 +1,13 @@
 a.external-link::after {
-    content: " [↪]";
+    /* \00A0 is a non-breaking space
+        to make the mark be on the same line as the link
+    */
+    content: "\00A0[↪]";
+}
+
+a.internal-link::after {
+    /* \00A0 is a non-breaking space
+        to make the mark be on the same line as the link
+    */
+    content: "\00A0↪";
 }