docs: new formatting syntax ideas

docs/users_guide/framework_basics/faster_than_lightspeed_constants.md

@@ -61,7 +61,7 @@ constexpr auto speed_of_light_in_vacuum = 1 * si::si2019::speed_of_light_in_vacu
 
 QuantityOf<isq::permittivity_of_vacuum> auto q = 1 / (permeability_of_vacuum * pow<2>(speed_of_light_in_vacuum));
 
-std::println("permittivity of vacuum = {} = {:{%N:.3e} %U}", q, q.in(F / m));
+std::println("permittivity of vacuum = {} = {::N[.3e]}", q, q.in(F / m));
 ```
 
 The above first prints the following:

docs/users_guide/framework_basics/simple_and_typed_quantities.md

@@ -75,7 +75,7 @@ Here is a simple example showing how to deal with such quantities:
       const quantity duration = 2 * h;
       const quantity speed = avg_speed(distance, duration);
 
-      std::println("A car driving {} in {} has an average speed of {:{%N:.4} %U} ({:{%N:.4} %U})",
+      std::println("A car driving {} in {} has an average speed of {::N[.4]} ({::N[.4]})",
                    distance, duration, speed, speed.in(km / h));
     }
     ```
@@ -103,7 +103,7 @@ Here is a simple example showing how to deal with such quantities:
       const quantity duration = 2 * h;
       const quantity speed = avg_speed(distance, duration);
 
-      std::println("A car driving {} in {} has an average speed of {:{%N:.4} %U} ({:{%N:.4} %U})",
+      std::println("A car driving {} in {} has an average speed of {::N[.4]} ({::N[.4]})",
                    distance, duration, speed, speed.in(km / h));
     }
     ```
@@ -194,7 +194,7 @@ The previous example can be re-typed using typed quantities in the following way
       const quantity duration = isq::time(2 * h);
       const quantity speed = avg_speed(distance, duration);
 
-      std::println("A car driving {} in {} has an average speed of {:{%N:.4} %U} ({:{%N:.4} %U})",
+      std::println("A car driving {} in {} has an average speed of {::N[.4]} ({::N[.4]})",
                    distance, duration, speed, speed.in(km / h));
     }
     ```
@@ -223,7 +223,7 @@ The previous example can be re-typed using typed quantities in the following way
       const quantity duration = isq::time(2 * h);
       const quantity speed = avg_speed(distance, duration);
 
-      std::println("A car driving {} in {} has an average speed of {:{%N:.4} %U} ({:{%N:.4} %U})",
+      std::println("A car driving {} in {} has an average speed of {::N[.4]} ({::N[.4]})",
                    distance, duration, speed, speed.in(km / h));
     }
     ```

docs/users_guide/framework_basics/the_affine_space.md

@@ -501,7 +501,7 @@ std::println("| {:<18} | {:^18} | {:^18} | {:^18} |",
 std::println("|{0:=^20}|{0:=^20}|{0:=^20}|{0:=^20}|", "");
 
 auto print_temp = [&](std::string_view label, auto v) {
-  std::println("| {:<18} | {:^18} | {:^18} | {:^18{%N:.2f}%?%U} |", label,
+  std::println("| {:<14} | {:^18} | {:^18} | {:^18:N[.2f]} |", label,
                v - room_reference_temp, (v - si::ice_point).in(deg_C), (v - si::absolute_zero).in(deg_C));
 };