📝 add more API documentation

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

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+# basic_json::emplace_back
+
+```cpp
+template<class... Args>
+reference emplace_back(Args&& ... args);
+```
+
+Creates a JSON value from the passed parameters `args` to the end of the JSON value. If the function is called on a JSON
+`#!json null` value, an empty array is created before appending the value created from `args`.
+
+## Template parameters
+
+`Args`
+:   compatible types to create a `basic_json` object
+
+## Parameters
+
+`args` (in)
+:   arguments to forward to a constructor of `basic_json`
+
+## Return value
+
+reference to the inserted element
+
+## Exceptions
+
+Throws [`type_error.311`](../../home/exceptions.md#jsonexceptiontype_error311) when called on a type other than JSON
+array or `#!json null`; example: `"cannot use emplace_back() with number"`
+
+## Complexity
+
+Amortized constant.
+
+## Examples
+
+??? example
+
+    The example shows how `emplace_back()` can be used to add elements to a JSON array. Note how the `null` value was
+    silently converted to a JSON array.
+        
+    ```cpp
+    --8<-- "examples/emplace_back.cpp"
+    ```
+    
+    Output:
+    
+    ```json
+    --8<-- "examples/emplace_back.output"
+    ```
+
+## Version history
+
+- Since version 2.0.8.
+- Returns reference since 3.7.0.