📝 add more API documentation

doc/mkdocs/docs/api/basic_json/operator!=.md

@@ -0,0 +1,57 @@
+# 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 two JSON values for inequality by calculating `#!cpp !(lhs == rhs)`.
+
+## 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 the values `lhs` and `rhs` are not equal
+
+## Exception safety
+
+No-throw guarantee: this function never throws exceptions.
+
+## Complexity
+
+Linear.
+
+## Example
+
+The example demonstrates comparing several JSON
+types.
+    
+```cpp
+--8<-- "examples/operator__notequal.cpp"
+```
+
+Output:
+
+```json
+--8<-- "examples/operator__notequal.output"
+```
+
+## Version history
+
+- Added in version 1.0.0.