📝 add more API documentation

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

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+# basic_json::emplace
+
+```cpp
+template<class... Args>
+std::pair<iterator, bool> emplace(Args&& ... args);
+```
+
+Inserts a new element into a JSON object constructed in-place with the given `args` if there is no element with the key
+in the container. If the function is called on a JSON null value, an empty object 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
+
+a pair consisting of an iterator to the inserted element, or the already-existing element if no insertion happened, and
+a `#!cpp bool` denoting whether the insertion took place.
+
+## Exceptions
+
+Throws [`type_error.311`](../../home/exceptions.md#jsonexceptiontype_error311) when called on a type other than JSON
+object or `#!json null`; example: `"cannot use emplace() with number"`
+
+## Complexity
+
+Logarithmic in the size of the container, O(log(`size()`)).
+
+## Examples
+
+??? example
+
+    The example shows how `emplace()` can be used to add elements to a JSON object. Note how the `#!json null` value was
+    silently converted to a JSON object. Further note how no value is added if there was already one value stored with
+    the same key.
+            
+    ```cpp
+    --8<-- "examples/emplace.cpp"
+    ```
+    
+    Output:
+    
+    ```json
+    --8<-- "examples/emplace.output"
+    ```
+
+## Version history
+
+- Since version 2.0.8.