docs: "Quick Start" chapter reworked to be simpler and include quanti…

…ty points

docs/getting_started/quick_start.md

@@ -1,10 +1,12 @@
 # Quick Start
 
-A **quantity** is a concrete amount of a unit representing a quantity type of a specified dimension with a
-specific representation. It is represented in the library with a `quantity` class template.
+This chapter provides a quick introduction to get you started with **mp-units**.
+Much more details can be found in our [User's Guide](../users_guide/terms_and_definitions.md).
 
+## Quantities
 
-## Creating a quantity
+A **quantity** is a concrete amount of a unit representing a quantity type of a specified dimension with a
+specific representation. It is represented in the library with a `quantity` class template.
 
 The [SI Brochure](../appendix/references.md#SIBrochure) says:
 
@@ -23,24 +25,33 @@ a number with a predefined unit:
 
 using namespace mp_units;
 
-quantity q = 42 * si::metre;
+quantity q = 42 * si::metre / si::second;
 ```
 
-!!! note
+!!! info
+
+    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 two-parameter
+    constructor:
 
-    The above spelling of `metre` is not a typo. For motivation, please check our
-    [FAQ](faq.md#why-do-we-spell-metre-instead-of-meter).
+    ```cpp
+    #include <mp-units/systems/si/si.h>
 
-The above creates an instance of `quantity<si::metre(), int>`. The same can be obtained using
-an optional unit symbol:
+    using namespace mp_units;
+
+    quantity q{42, si::metre / si::second};
+    ```
+
+The above creates an instance of `quantity<derived_unit<si::metre, per<si::second>>{}, int>`.
+The same can be obtained using optional unit symbols:
 
 ```cpp
 #include <mp-units/systems/si/si.h>
 
 using namespace mp_units;
 using namespace mp_units::si::unit_symbols;
 
-quantity q = 42 * m;
+quantity q = 42 * m / s;
 ```
 
 !!! tip
@@ -49,36 +60,55 @@ quantity q = 42 * m;
     why they are opt-in. A user has to explicitly "import" them from a dedicated `unit_symbols`
     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 two-parameter
-constructor:
+Quantities of the same kind can be added, subtracted, and compared to each other:
 
 ```cpp
 #include <mp-units/systems/si/si.h>
 
 using namespace mp_units;
+using namespace mp_units::si::unit_symbols;
 
-quantity q{42, si::metre};
+static_assert(1 * km + 50 * m == 1050 * m);
 ```
 
-
-## User-provided unit wrappers
-
-Sometimes it might be awkward to type some derived units:
+Various quantities can be multiplied or divided by each other:
 
 ```cpp
-quantity speed = 60 * km / h;
+static_assert(140 * km / (2 * h) == 70 * km / h);
 ```
 
-In case such a unit is used a lot in the project, a user can easily provide a nicely named
-wrapper for it with:
+!!! note
+
+    In case you wonder why this library does not use UDLs to create quantities, please check
+    our [FAQ](faq.md#why-dont-we-use-udls-to-create-quantities).
+
+
+## Quantity points
+
+The `quantity_point` class template specifies an absolute quantity with respect to an origin.
+Together with quantities they model [The Affine Space](../users_guide/framework_basics/the_affine_space.md).
 
 ```cpp
-constexpr auto kmph = km / h;
-quantity speed = 60 * kmph;
+#include <mp-units/ostream.h>
+#include <mp-units/systems/si/si.h>
+#include <mp-units/systems/usc/usc.h>
+#include <iostream>
+
+int main()
+{
+  using namespace mp_units;
+  using namespace mp_units::si::unit_symbols;
+  using namespace mp_units::usc::unit_symbols;
+
+  quantity_point temp = si::zeroth_degree_Celsius + 20. * deg_C;
+  std::cout << "Temperature: "
+            << temp.quantity_from(si::zeroth_degree_Celsius) << " ("
+            << temp.in(deg_F).quantity_from(usc::zeroth_degree_Fahrenheit) << ")\n";
+}
 ```
 
-!!! note
+The above outputs:
 
-    In case you wonder why this library does not use UDLs to create quantities, please check
-    our [FAQ](faq.md#why-dont-we-use-udls-to-create-quantities).
+```text
+Temperature: 20 °C (68 °F)
+```

docs/users_guide/framework_basics/simple_and_typed_quantities.md

@@ -89,6 +89,23 @@ A car driving 110 km in 2 h has an average speed of 15.2778 m/s (55 km/h)
 !!! example "[Try it on Compiler Explorer](https://godbolt.org/z/zWe8ecf93)"
 
 
+### User-provided unit wrappers
+
+Sometimes it might be awkward to type some derived units:
+
+```cpp
+quantity speed = 60 * km / h;
+```
+
+In case such a unit is used a lot in the project, a user can easily provide a nicely named
+wrapper for it with:
+
+```cpp
+constexpr auto kmph = km / h;
+quantity speed = 60 * kmph;
+```
+
+
 ### Easy-to-understand compilation error messages
 
 In case a user makes an error in a quantity equation and the result of the calculation