📝 add more API documentation

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

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+# basic_json::number_float_t
+
+```cpp
+using number_float_t = NumberFloatType;
+```
+
+The type used to store JSON numbers (floating-point).
+
+[RFC 7159](http://rfc7159.net/rfc7159) describes numbers as follows:
+> The representation of numbers is similar to that used in most programming languages. A number is represented in base
+> 10 using decimal digits. It contains an integer component that may be prefixed with an optional minus sign, which may
+> be followed by a fraction part and/or an exponent part. Leading zeros are not allowed. (...) Numeric values that
+> cannot be represented in the grammar below (such as Infinity and NaN) are not permitted.
+
+This description includes both integer and floating-point numbers. However, C++ allows more precise storage if it is
+known whether the number is a signed integer, an unsigned integer or a floating-point number. Therefore, three different
+types, [`number_integer_t`](number_integer_t.md), [`number_unsigned_t`](number_unsigned_t.md) and `number_float_t` are
+used.
+
+To store floating-point numbers in C++, a type is defined by the template parameter `NumberFloatType` which chooses the
+type to use.
+
+## Notes
+
+#### Default type
+
+With the default values for `NumberFloatType` (`double`), the default value for `number_float_t` is `#!cpp double`.
+
+#### Default behavior
+
+- The restrictions about leading zeros is not enforced in C++. Instead, leading zeros in floating-point literals will be
+  ignored. Internally, the value will be stored as decimal number. For instance, the C++ floating-point literal `01.2`
+  will be serialized to `1.2`. During deserialization, leading zeros yield an error.
+- Not-a-number (NaN) values will be serialized to `null`.
+
+#### Limits
+
+[RFC 7159](http://rfc7159.net/rfc7159) states:
+> This specification allows implementations to set limits on the range and precision of numbers accepted. Since software
+> that implements IEEE 754-2008 binary64 (double precision) numbers is generally available and widely used, good
+> interoperability can be achieved by implementations that expect no more precision or range than these provide, in the
+> sense that implementations will approximate JSON numbers within the expected precision.
+
+This implementation does exactly follow this approach, as it uses double precision floating-point numbers. Note values
+smaller than `-1.79769313486232e+308` and values greater than `1.79769313486232e+308` will be stored as NaN internally
+and be serialized to `null`.
+
+#### Storage
+
+Floating-point number values are stored directly inside a `basic_json` type.
+
+## Version history
+
+- Added in version 1.0.0.