refactor: References are now guarded UNITS_REFERENCES with (ON by default) + examples duplicated to subdirectories

Now References can be disabled to meassure a compile time impact. Also the same examples are now provided in two subdirectories to be able to easily compare the pros and cons of every quantity construction technique.

docs/framework/quantities.rst

@@ -56,6 +56,8 @@ Quantity References
 Quantity References provide an alternative and simplified way to create quantities.
 They are defined using the `reference` class template::
 
+    #ifdef UNITS_REFERENCES
+
     namespace references {
 
     inline constexpr auto km = reference<dim_length, kilometre>{};
@@ -63,6 +65,8 @@ They are defined using the `reference` class template::
 
     }
 
+    #endif // UNITS_REFERENCES
+
 With the above our code can look as follows::
 
     using namespace units::isq::si::references;
@@ -103,8 +107,6 @@ Alternatively, to construct quantities with compile-time known values the librar
 
 Thanks to them the same code can be as simple as::
 
-    #define UNITS_LITERALS
-
     using namespace units::isq::si::literals;
     auto d = 123._q_km;     // si::length<si::kilometre, long double>
     auto v = 70_q_km_per_h; // si::speed<si::kilometre_per_hour, std::int64_t>
@@ -122,7 +124,7 @@ Thanks to them the same code can be as simple as::
 
     As one can read in the next section UDLs, are considered to be inferior to `Quantity References`_
     and their definition affects compile-time performance a lot. This is why they are an opt-in feature
-    of the library and in order to use them one has to provide a `UNITS_UDL` preprocessor definition.
+    of the library and in order to use them one has to provide a `UNITS_LITERALS` preprocessor definition.
 
     
 UDLs vs Quantity References