Reorganize directories (#3462)

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

docs/mkdocs/docs/api/json_sax/null.md

@@ -0,0 +1,15 @@
+# <small>nlohmann::json_sax::</small>null
+
+```cpp
+virtual bool null() = 0;
+```
+
+A null value was read.
+
+## Return value
+
+Whether parsing should proceed.
+
+## Version history
+
+- Added in version 3.2.0.