Reorganize directories (#3462)

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

docs/mkdocs/docs/api/basic_json/get_ptr.md

@@ -0,0 +1,60 @@
+# <small>nlohmann::basic_json::</small>get_ptr
+
+```cpp
+template<typename PointerType>
+PointerType get_ptr() noexcept;
+
+template<typename PointerType>
+constexpr const PointerType get_ptr() const noexcept;
+```
+
+Implicit pointer access to the internally stored JSON value. No copies are made.
+
+## Template parameters
+
+`PointerType`
+:   pointer type; must be a pointer to [`array_t`](array_t.md), [`object_t`](object_t.md), [`string_t`](string_t.md),
+    [`boolean_t`](boolean_t.md), [`number_integer_t`](number_integer_t.md), or
+    [`number_unsigned_t`](number_unsigned_t.md), [`number_float_t`](number_float_t.md), or [`binary_t`](binary_t.md).
+    Other types will not compile.
+
+## Return value
+
+pointer to the internally stored JSON value if the requested pointer type fits to the JSON value; `#!cpp nullptr`
+otherwise
+
+## Exception safety
+
+No-throw guarantee: this function never throws exceptions.
+
+## Complexity
+
+Constant.
+
+## Notes
+
+!!! danger "Undefined behavior"
+
+    Writing data to the pointee of the result yields an undefined state.
+
+## Examples
+
+??? example
+
+    The example below shows how pointers to internal values of a JSON value can be requested. Note that no type
+    conversions are made and a `#!cpp nullptr` is returned if the value and the requested pointer type does not match.
+    
+    ```cpp
+    --8<-- "examples/get_ptr.cpp"
+    ```
+    
+    Output:
+    
+    ```json
+    --8<-- "examples/get_ptr.output"
+    ```
+
+## Version history
+
+- Added in version 1.0.0.
+- Extended to binary types in version 3.8.0.