Reorganize directories (#3462)

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

docs/mkdocs/docs/api/basic_json/exception.md

@@ -0,0 +1,75 @@
+# <small>nlohmann::basic_json::</small>exception
+
+```cpp
+class exception : public std::exception;
+```
+
+This class is an extension of [`std::exception`](https://en.cppreference.com/w/cpp/error/exception) objects with a
+member `id` for exception ids. It is used as the base class for all exceptions thrown by the `basic_json` class. This
+class can hence be used as "wildcard" to catch exceptions, see example below.
+
+```plantuml
+std::exception <|-- basic_json::exception
+basic_json::exception <|-- basic_json::parse_error
+basic_json::exception <|-- basic_json::invalid_iterator
+basic_json::exception <|-- basic_json::type_error
+basic_json::exception <|-- basic_json::out_of_range
+basic_json::exception <|-- basic_json::other_error
+
+interface std::exception {}
+
+class basic_json::exception #FFFF00 {
+    + const int id
+    + const char* what() const
+}
+
+class basic_json::parse_error {
+    + const std::size_t byte
+}
+```
+
+Subclasses:
+
+- [`parse_error`](parse_error.md) for exceptions indicating a parse error
+- [`invalid_iterator`](invalid_iterator.md) for exceptions indicating errors with iterators
+- [`type_error`](type_error.md) for exceptions indicating executing a member function with a wrong type
+- [`out_of_range`](out_of_range.md) for exceptions indicating access out of the defined range
+- [`other_error`](other_error.md) for exceptions indicating other library errors
+
+## Member functions
+
+- **what** - returns explanatory string
+
+## Member variables
+
+- **id** - the id of the exception
+
+## Notes
+
+To have nothrow-copy-constructible exceptions, we internally use `std::runtime_error` which can cope with
+arbitrary-length error messages. Intermediate strings are built with static functions and then passed to the actual
+constructor.
+
+## Examples
+
+??? example
+
+    The following code shows how arbitrary library exceptions can be caught.
+    
+    ```cpp
+    --8<-- "examples/exception.cpp"
+    ```
+    
+    Output:
+    
+    ```json
+    --8<-- "examples/exception.output"
+    ```
+
+## See also
+
+[List of exceptions](127.0.0.1:8000/home/exceptions/)
+
+## Version history
+
+- Since version 3.0.0.