Sphinx-based documentation added

docs/framework/text_output.rst

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+.. namespace:: units
+
+Text output
+===========
+
+Beside providing dimensional analysis and units conversions, the library
+also tries really hard to print any quantity in the most user friendly way.
+
+Output streams
+--------------
+
+The easiest way to print a quantity is to provide its object to the output
+stream::
+
+    using namespace si::literals;
+    using namespace international::literals;
+    constexpr Velocity auto v1 = avg_speed(220.q_km, 2q_h);
+    constexpr Velocity auto v2 = avg_speed(140.q_mi, 2q_h);
+    std::cout << v1 << '\n';  // 110 km/h
+    std::cout << v2 << '\n';  // 70 mi/h
+
+The text output will always print the :term:`value of a quantity` followed
+by the symbol of a :term:`unit` associated with this quantity. We will learn
+more about units in the :ref:`Units` chapter, but for now, it is important
+to remember that it is a good practice to always `quantity_cast()` a quantity
+of an unknown ``auto`` type before passing it to the text output::
+
+    std::cout << quantity_cast<si::kilometre_per_hour>(v1) << '\n';  // 110 km/h
+    std::cout << quantity_cast<si::metre_per_second>(v1) << '\n';    // 30.5556 m/s
+
+
+Formatting the output
+---------------------
+
+Grammar:
+
+.. productionlist::
+    units-format-spec: fill-and-align[opt] sign[opt] width[opt] precision[opt] units-specs[opt]
+    units-specs: conversion-spec
+               : units-specs conversion-spec
+               : units-specs literal-char
+    literal-char: any character other than '{' or '}'
+    conversion-spec: '%' modifier[opt] type
+    modifier: one of 'E', 'O'
+    type: one of 'n', 'q', 'Q', 't', '%'