Reorganize directories (#3462)

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

docs/mkdocs/docs/api/basic_json/string_t.md

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+# <small>nlohmann::basic_json::</small>string_t
+
+```cpp
+using string_t = StringType;
+```
+
+The type used to store JSON strings.
+
+[RFC 8259](https://tools.ietf.org/html/rfc8259) describes JSON strings as follows:
+> A string is a sequence of zero or more Unicode characters.
+
+To store objects in C++, a type is defined by the template parameter described below. Unicode values are split by the
+JSON class into byte-sized characters during deserialization.
+
+## Template parameters
+
+`StringType`
+:   the container to store strings (e.g., `std::string`). Note this container is used for keys/names in objects, see
+    [object_t](object_t.md).
+
+## Notes
+
+#### Default type
+
+With the default values for `StringType` (`std::string`), the default value for `string_t` is `#!cpp std::string`.
+
+#### Encoding
+
+Strings are stored in UTF-8 encoding. Therefore, functions like `std::string::size()` or `std::string::length()` return
+the number of bytes in the string rather than the number of characters or glyphs.
+
+#### String comparison
+
+[RFC 8259](https://tools.ietf.org/html/rfc8259) states:
+> Software implementations are typically required to test names of object members for equality. Implementations that
+> transform the textual representation into sequences of Unicode code units and then perform the comparison numerically,
+> code unit by code unit, are interoperable in the sense that implementations will agree in all cases on equality or
+> inequality of two strings. For example, implementations that compare strings with escaped characters unconverted may
+> incorrectly find that `"a\\b"` and `"a\u005Cb"` are not equal.
+
+This implementation is interoperable as it does compare strings code unit by code unit.
+
+#### Storage
+
+String values are stored as pointers in a `basic_json` type. That is, for any access to string values, a pointer of type
+`string_t*` must be dereferenced.
+
+## Examples
+
+??? example
+
+    The following code shows that `string_t` is by default, a typedef to `#!cpp std::string`.
+     
+    ```cpp
+    --8<-- "examples/string_t.cpp"
+    ```
+    
+    Output:
+    
+    ```json
+    --8<-- "examples/string_t.output"
+    ```
+
+## Version history
+
+- Added in version 1.0.0.