Reorganize directories (#3462)

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

docs/mkdocs/docs/api/json_sax/start_object.md

@@ -0,0 +1,24 @@
+# <small>nlohmann::json_sax::</small>start_object
+
+```cpp
+virtual bool start_object(std::size_t elements) = 0;
+```
+
+The beginning of an object was read.
+
+## Parameters
+
+`elements` (in)
+:   number of object elements or `#!cpp -1` if unknown
+
+## Return value
+
+Whether parsing should proceed.
+
+## Notes
+
+Binary formats may report the number of elements.
+
+## Version history
+
+- Added in version 3.2.0.