📝 add more API documentation

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

@@ -0,0 +1,26 @@
+# basic_json::boolean_t
+
+```cpp
+using boolean_t = BooleanType;
+```
+
+The type used to store JSON booleans.
+
+[RFC 7159](http://rfc7159.net/rfc7159) implicitly describes a boolean as a type which differentiates the two literals
+`#!json true` and `#!json false`.
+
+To store objects in C++, a type is defined by the template parameter  `BooleanType` which chooses the type to use.
+
+## Notes
+
+#### Default type
+
+With the default values for `BooleanType` (`#!cpp bool`), the default value for `boolean_t` is `#!cpp bool`.
+
+#### Storage
+
+Boolean values are stored directly inside a `basic_json` type.
+
+## Version history
+
+- Added in version 1.0.0.