📝 add more API documentation

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

@@ -0,0 +1,60 @@
+# basic_json::is_primitive
+
+```cpp
+constexpr bool is_primitive() const noexcept;
+```
+
+This function returns `#!cpp true` if and only if the JSON type is primitive (string, number, boolean, `#!json null`,
+binary).
+    
+## Return value
+
+`#!cpp true` if type is primitive (string, number, boolean, `#!json null`, or binary), `#!cpp false` otherwise.
+
+## Exception safety
+
+No-throw guarantee: this member function never throws exceptions.
+
+## Complexity
+
+Constant.
+
+## Possible implementation
+
+```cpp
+constexpr bool is_primitive() const noexcept
+{
+    return is_null() || is_string() || is_boolean() || is_number() || is_binary();
+}
+```
+
+## Notes
+
+The term *primitive* stems from [RFC 8259](https://tools.ietf.org/html/rfc8259):
+
+> JSON can represent four primitive types (strings, numbers, booleans, and null) and two structured types (objects and
+> arrays).
+
+This library extends primitive types to binary types, because binary types are  roughly comparable to strings. Hence,
+`is_primitive()` returns `#!cpp true` for binary values.
+
+## Example
+
+??? example
+
+    The following code exemplifies `is_primitive()` for all JSON types.
+    
+    ```cpp
+    --8<-- "examples/is_primitive.cpp"
+    ```
+    
+    Output:
+    
+    ```json
+    --8<-- "examples/is_primitive.output"
+    ```
+
+## Version history
+
+- Added in version 1.0.0.
+- Extended to return `#!cpp true` for binary types in version 3.8.0.