refactor: 💥 make_xxx factory functions replaced with two-parameter constructors

Resolves #521

docs/getting_started/quick_start.md

@@ -50,15 +50,15 @@ quantity q = 42 * m;
     namespace.
 
 In case someone doesn't like the multiply syntax or there is an ambiguity between `operator*`
-provided by this and other libraries, a quantity can also be created with a dedicated factory
-function:
+provided by this and other libraries, a quantity can also be created with a two-parameter
+constructor:
 
 ```cpp
 #include <mp-units/systems/si/si.h>
 
 using namespace mp_units;
 
-quantity q = make_quantity<si::metre>(42);
+quantity q{42, si::metre};
 ```
 
 

docs/users_guide/framework_basics/character_of_a_quantity.md

@@ -191,10 +191,10 @@ Quantity auto q = la_vector{1, 2, 3} * isq::velocity[m / s];
 In case there is an ambiguity of `operator*` between **mp-units** and a linear algebra library, we can
 either:
 
-- use `make_quantity` factory function
+- use two-parameter constructor
 
     ```cpp
-    Quantity auto q = make_quantity<isq::velocity[m / s]>(la_vector{1, 2, 3});
+    Quantity auto q = quantity{la_vector{1, 2, 3}, isq::velocity[m / s]};
     ```
 
 - provide a dedicated overload of `operator*` that will resolve the ambiguity and wrap the above
@@ -203,7 +203,7 @@ either:
     template<Reference R>
     Quantity auto operator*(la_vector rep, R)
     {
-      return make_quantity<R{}>(rep);
+      return quantity{rep, R{}};
     }
     ```
 

docs/users_guide/framework_basics/the_affine_space.md

@@ -102,11 +102,11 @@ quantity_point qp3 = mean_sea_level - 42 * m;
 
 Similarly to [creation of a quantity](../../getting_started/quick_start.md#creating-a-quantity),
 if someone does not like the operator-based syntax to create a `quantity_point`, the same results
-can be achieved with `make_quantity_point` factory function:
+can be achieved with two-parameter constructor:
 
 ```cpp
-quantity_point qp4 = make_quantity_point<mean_sea_level>(42 * m);
-quantity_point qp5 = make_quantity_point<mean_sea_level>(-42 * m);
+quantity_point qp4{42 * m, mean_sea_level};
+quantity_point qp5{-42 * m, mean_sea_level};
 ```
 
 The provided `quantity` representing an offset from the origin is stored inside the `quantity_point`