docs: add section on quantity points

docs/framework/quantity_points.rst

@@ -0,0 +1,38 @@
+.. namespace:: units
+
+Quantity Points
+===============
+
+A :term:`quantity point` is an absolute quantity with respect to zero
+(which represents some origin) and is represented in the library with a
+`quantity_point` class template.
+
+
+Construction
+------------
+
+To create the quantity point object from a `quantity` we just have to pass
+the value to the `quantity_point` class template explicit constructor::
+
+    quantity_point<si::dim_length, si::kilometre, double> d(123q_km);
+
+.. note::
+
+    As the constructor is explicit, the quantity object can be created from
+    a quantity only via
+    `direct initialization <https://en.cppreference.com/w/cpp/language/direct_initialization>`_.
+    This is why the code below using
+    `copy initialization <https://en.cppreference.com/w/cpp/language/copy_initialization>`_
+    **does not compile**::
+
+        quantity_point<si::dim_length, si::kilometre, double> d = 123q_km;  // ERROR
+
+
+Differences to quantity
+-----------------------
+
+Unlike `quantity`, the library provides
+- no helper aliases for quantity points, such as `length_point`,
+- no UDLs for quantity points,
+- no dimension-specific concepts, such as `LengthPoint`
+  (there's the dimension-agnostic `QuantityPoint`),