docs: initial V2 documenatation added

docs/getting_started/quick_start.md

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+# Quick Start
+
+A **quantity** is a concrete amount of a unit for a quantity type of a specified dimension with a
+specific representation, and is represented in the library with a `quantity` class template.
+
+
+## Creating a quantity
+
+The quantity is created by multiplying a number with a predefined unit:
+
+```cpp
+#include <mp-units/systems/si/si.h>
+
+using namespace mp_units;
+
+auto q = 42 * si::metre;
+```
+
+!!! note
+
+    The above spelling of `metre` is not a typo. For motivation, please check our
+    [FAQ](../faq/#why-do-we-spell-metre-instead-of-meter).
+
+The above creates an instance of `quantity<si::metre(), int>`. The same can be obtained using
+an optional unit symbol:
+
+```cpp
+#include <mp-units/systems/si/si.h>
+
+using namespace mp_units;
+using namespace mp_units::si::unit_symbols;
+
+auto q = 42 * m;
+```
+
+!!! note
+
+    Unit symbols introduce a lot of short identifiers into the current namespace, and that is
+    why they are opt-in. A user has to explicitly "import" them from a dedicated `unit_symbols`
+    namespace.
+
+
+## User-provided unit wrappers
+
+Sometimes it might be awkward to type some derived units:
+
+```cpp
+auto speed = 60 * (km / h);
+```
+
+!!! note
+
+    Please note that `60 * km / h` will not compile. To read more about the rationale for such
+    a design please check our [FAQ](../faq/#why-dont-we-use-udls-to-create-a-quantity).
+
+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;
+auto speed = 60 * kmph;
+```
+
+or even:
+
+```cpp
+constexpr auto kilometre = si::kilo<si::metre>;
+constexpr auto kilometre_per_hour = kilometre / si::hour;
+constexpr auto kmph = kilometre_per_hour;
+auto speed = 60 * kmph;
+```
+
+!!! note
+
+    In case you wonder why this library does not use UDLs to create quantities, please check
+    our [FAQ](../faq/#why-dont-we-use-udls-to-create-quantities).