📝 add more API documentation

doc/mkdocs/docs/api/basic_json/value_t.md

@@ -0,0 +1,40 @@
+# basic_json::value_t
+
+```cpp
+enum class value_t : std::uint8_t {
+    null,
+    object,
+    array,
+    string,
+    boolean,
+    number_integer,
+    number_unsigned,
+    number_float,
+    binary,
+    discarded
+};
+```
+
+This enumeration collects the different JSON types. It is internally used to
+distinguish the stored values, and the functions [`is_null`](is_null.md),
+[`is_object`](is_object.md), [`is_array`](is_array.md),
+[`is_string`](is_string.md), [`is_boolean`](is_boolean.md),
+[`is_number`](is_number.md) (with [`is_number_integer`](is_number_integer.md),
+[`is_number_unsigned`](is_number_unsigned.md), and [`is_number_float`](is_number_float.md)),
+[`is_discarded`](is_discarded.md), [`is_binary`](is_binary.md), [`is_primitive`](is_primitive.md), and
+[`is_structured`](is_structured.md) rely on it.
+
+## Note
+
+There are three enumeration entries (number_integer, number_unsigned, and
+number_float), because the library distinguishes these three types for numbers:
+[`number_unsigned_t`](number_unsigned_t.md) is used for unsigned integers,
+[`number_integer_t`](number_integer_t.md) is used for signed integers, and
+[`number_float_t`](number_float_t.md) is used for floating-point numbers or to
+approximate integers which do not fit in the limits of their respective type.
+
+## Version history
+
+- Added in version 1.0.0.
+- Added unsigned integer type in version 2.0.0.
+- Added binary type in version 3.8.0.