feat: common_unit support added

docs/users_guide/framework_basics/systems_of_units.md

@@ -260,3 +260,31 @@ This is why we provide both versions of identifiers for such units.
     quantity resistance = 60 * kΩ;
     quantity capacitance = 100 * µF;
     ```
+
+
+## Common units
+
+Adding or subtracting two quantities of different units will force the library to find a common
+unit for those. This is to prevent data truncation. For the cases when one of the units is an
+integral multiple of the another, the resulting quantity will use a "smaller" one in its result.
+For example:
+
+```cpp
+static_assert((1 * kg + 1 * g).unit == g);
+static_assert((1 * km + 1 * mm).unit == mm);
+static_assert((1 * yd + 1 * mi).unit == yd);
+```
+
+However, in many cases an arithmetic on quantities of different units will result in a yet another
+unit. This happens when none of the source units is an integral multiple of another. In such cases,
+the library returns a special type that denotes that we are dealing with a common unit of such
+an equation:
+
+```cpp
+quantity q = 1 * km + 1 * mi;  // quantity<common_unit<international::mile, si::kilo_<si::metre>>{}, int>
+```
+
+!!! note
+
+    A user should never explicitly instantiate a `common_unit` class template. The library's
+    framework will do it based on the provided quantity equation.

docs/users_guide/framework_basics/text_output.md

@@ -268,6 +268,31 @@ The above prints:
 kg⋅m⋅s⁻²
 ```
 
+## Symbols of common units
+
+Some [common units](systems_of_units.md#common-units) expressed with a specialization of the
+`common_unit` class template need special printing rules for their symbols. As they represent
+a minimum set of common units resulting from the addition or subtraction of multiple quantities,
+we print all of them as a scaled version of the source unit. For example the following:
+
+```cpp
+std::cout << 1 * km + 1 * mi << "\n";
+std::cout << 1 * nmi + 1 * mi << "\n";
+std::cout << 1 * km / h + 1 * m / s << "\n";
+```
+
+will print:
+
+```text
+40771 ([1/25146] mi = [1/15625] km)
+108167 ([1/50292] mi = [1/57875] nmi)
+23 ([1/5] km/h = [1/18] m/s)
+```
+
+Thanks to the above, it might be easier for the user to reason about the magnitude of the resulting
+unit and its impact on the value stored in the quantity.
+
+
 ## `space_before_unit_symbol` customization point
 
 The [SI Brochure](../../appendix/references.md#SIBrochure) says: