📝 add more API documentation

doc/mkdocs/docs/api/basic_json/operator<.md

@@ -0,0 +1,73 @@
+# basic_json::operator<
+
+```cpp
+bool operator<(const_reference lhs, const_reference rhs) noexcept,
+
+template<typename ScalarType>
+bool operator<(const_reference lhs, const ScalarType rhs) noexcept;
+
+template<typename ScalarType>
+bool operator<(ScalarType lhs, const const_reference rhs) noexcept;
+```
+
+Compares whether one JSON value `lhs` is less than another JSON value `rhs` according to the following rules:
+
+- If `lhs` and `rhs` have the same type, the values are compared using the default `<` operator.
+- Integer and floating-point numbers are automatically converted before comparison
+- Discarded values a
+- In case `lhs` and `rhs` have different types, the values are ignored and the order of the types is considered, which
+  is:
+    1. null
+    2. boolean
+    3. number (all types)
+    4. object
+    5. array
+    6. string
+    7. binary
+
+    For instance, any boolean value is considered less than any string.
+
+## Template parameters
+
+`ScalarType`
+:   a scalar type according to `std::is_scalar<ScalarType>::value`
+
+## Parameters
+
+`lhs` (in)
+:   first value to consider 
+
+`rhs` (in)
+:   second value to consider 
+
+## Return value
+
+whether `lhs` is less than `rhs`
+
+## Exception safety
+
+No-throw guarantee: this function never throws exceptions.
+
+## Complexity
+
+Linear.
+
+## Example
+
+??? example
+
+    The example demonstrates comparing several JSON types.
+        
+    ```cpp
+    --8<-- "examples/operator__less.cpp"
+    ```
+    
+    Output:
+    
+    ```json
+    --8<-- "examples/operator__less.output"
+    ```
+
+## Version history
+
+- Added in version 1.0.0.