📝 add more API documentation

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

@@ -0,0 +1,45 @@
+# basic_json::is_discarded
+
+```cpp
+constexpr bool is_discarded() const noexcept;
+```
+
+This function returns true if and only if the JSON value was discarded during parsing with a callback function (see
+[`parser_callback_t`](parser_callback_t.md)).
+    
+## Return value
+
+`#!cpp true` if type is discarded, `#!cpp false` otherwise.
+
+## Exception safety
+
+No-throw guarantee: this member function never throws exceptions.
+
+## Complexity
+
+Constant.
+
+## Notes
+
+This function will always be `#!cpp false` for JSON values after parsing. That is, discarded values can only occur
+during parsing, but will be removed when inside a structured value or replaced by null in other cases.
+
+## Example
+
+??? example
+
+    The following code exemplifies `is_discarded()` for all JSON types.
+    
+    ```cpp
+    --8<-- "examples/is_discarded.cpp"
+    ```
+    
+    Output:
+    
+    ```json
+    --8<-- "examples/is_discarded.output"
+    ```
+
+## Version history
+
+- Added in version 1.0.0.