feat: ppm parts per million

Co-authored-by: Mateusz Pusz <mateusz.pusz@gmail.com>

docs/users_guide/framework_basics/dimensionless_quantities.md

@@ -160,6 +160,8 @@ with dimensionless quantities:
 ```cpp
 inline constexpr struct percent : named_unit<"%", mag<ratio{1, 100}> * one> {} percent;
 inline constexpr struct per_mille : named_unit<basic_symbol_text{"‰", "%o"}, mag<ratio(1, 1000)> * one> {} per_mille;
+inline constexpr struct parts_per_million : named_unit<"ppm", mag<ratio(1, 1'000'000)> * one> {} parts_per_million;
+inline constexpr auto ppm = parts_per_million;
 ```