Reorganize directories (#3462)

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

docs/mkdocs/docs/api/json_sax/key.md

@@ -0,0 +1,24 @@
+# <small>nlohmann::json_sax::</small>key
+
+```cpp
+virtual bool key(string_t& val) = 0;
+```
+
+An object key was read.
+
+## Parameters
+
+`val` (in)
+:   object key
+
+## Return value
+
+Whether parsing should proceed.
+
+## Notes
+
+It is safe to move the passed object key value.
+
+## Version history
+
+- Added in version 3.2.0.