Reorganize directories (#3462)

* 🚚 move files
* 🚚 rename doc folder to docs
* 🚚 rename test folder to tests

docs/mkdocs/docs/api/basic_json/contains.md

@@ -0,0 +1,104 @@
+# <small>nlohmann::basic_json::</small>contains
+
+```cpp
+// (1)
+bool contains(const typename object_t::key_type& key) const;
+
+// (2)
+template<typename KeyType>
+bool contains(KeyType&& key) const;
+
+// (3)
+bool contains(const json_pointer& ptr) const;
+```
+
+1. Check whether an element exists in a JSON object with a key equivalent to `key`. If the element is not found or the 
+   JSON value is not an object, `#!cpp false` is returned.
+2. See 1. This overload is only available if `KeyType` is comparable with `#!cpp typename object_t::key_type` and
+   `#!cpp typename object_comparator_t::is_transparent` denotes a type.
+3. Check whether the given JSON pointer `ptr` can be resolved in the current JSON value.
+
+## Template parameters
+
+`KeyType`
+:   A type for an object key other than [`json_pointer`](../json_pointer/index.md) that is comparable with
+    [`string_t`](string_t.md) using  [`object_comparator_t`](object_comparator_t.md).
+    This can also be a string view (C++17).
+
+## Parameters
+
+`key` (in)
+:   key value to check its existence.
+
+`ptr` (in)
+:   JSON pointer to check its existence.
+
+## Return value
+
+1. `#!cpp true` if an element with specified `key` exists. If no such element with such key is found or the JSON value
+   is not an object, `#!cpp false` is returned.
+2. See 1.
+3. `#!cpp true` if the JSON pointer can be resolved to a stored value, `#!cpp false` otherwise.
+
+## Exception safety
+
+Strong exception safety: if an exception occurs, the original value stays intact.
+
+## Exceptions
+
+1. The function does not throw exceptions.
+2. The function does not throw exceptions.
+3. The function can throw the following exceptions:
+    - Throws [`parse_error.106`](../../home/exceptions.md#jsonexceptionparse_error106) if an array index begins with
+      `0`.
+    - Throws [`parse_error.109`](../../home/exceptions.md#jsonexceptionparse_error109) if an array index was not a
+      number.
+
+## Complexity
+
+Logarithmic in the size of the JSON object.
+
+## Notes
+
+1. This method always returns `#!cpp false` when executed on a JSON type that is not an object.
+2. This method can be executed on any JSON value type.
+
+!!! info "Postconditions"
+
+    If `#!cpp j.contains(x)` returns `#!c true` for a key or JSON pointer `x`, then it is safe to call `j[x]`.
+
+## Examples
+
+??? example "Example (1) check with key"
+
+    The example shows how `contains()` is used.
+    
+    ```cpp
+    --8<-- "examples/contains.cpp"
+    ```
+    
+    Output:
+    
+    ```json
+    --8<-- "examples/contains.output"
+    ```
+
+??? example "Example (3) check with JSON pointer"
+
+    The example shows how `contains()` is used.
+    
+    ```cpp
+    --8<-- "examples/contains_json_pointer.cpp"
+    ```
+    
+    Output:
+    
+    ```json
+    --8<-- "examples/contains_json_pointer.output"
+    ```
+
+## Version history
+
+1. Added in version 3.11.0.
+2. Added in version 3.6.0. Extended template `KeyType` to support comparable types in version 3.11.0.
+3. Added in version 3.7.0.