docs: new formatting syntax ideas

docs/getting_started/look_and_feel.md

@@ -100,8 +100,8 @@ performed without sacrificing accuracy. Please see the below example for a quick
       std::cout << std::setw(10) << std::setfill('*') << v2 << '\n';  // ***70 mi/h
       std::cout << std::format("{:*^10}\n", v3);                      // *110 km/h*
       std::println("{:%N in %U of %D}", v4);                          // 70 in mi/h of LT⁻¹
-      std::println("{:{%N:.2f}%?%U}", v5);                            // 30.56 m/s
-      std::println("{:{%N:.2f}%?{%U:dn}}", v6);                       // 31.29 m⋅s⁻¹
+      std::println("{::N[.2f]}", v5);                                 // 30.56 m/s
+      std::println("{::N[.2f]U[dn]}", v6);                            // 31.29 m⋅s⁻¹
       std::println("{:%N}", v7);                                      // 31
     }
     ```
@@ -144,8 +144,8 @@ performed without sacrificing accuracy. Please see the below example for a quick
       std::cout << std::setw(10) << std::setfill('*') << v2 << '\n';  // ***70 mi/h
       std::cout << std::format("{:*^10}\n", v3);                      // *110 km/h*
       std::println("{:%N in %U of %D}", v4);                          // 70 in mi/h of LT⁻¹
-      std::println("{:{%N:.2f}%?%U}", v5);                            // 30.56 m/s
-      std::println("{:{%N:.2f}%?{%U:dn}}", v6);                       // 31.29 m⋅s⁻¹
+      std::println("{::N[.2f]}", v5);                                 // 30.56 m/s
+      std::println("{::N[.2f]U[dn]}", v6);                            // 31.29 m⋅s⁻¹
       std::println("{:%N}", v7);                                      // 31
     }
     ```