Reorganize directories (#3462)

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

docs/mkdocs/docs/api/macros/json_has_filesystem.md

@@ -0,0 +1,30 @@
+# JSON_HAS_FILESYSTEM / JSON_HAS_EXPERIMENTAL_FILESYSTEM
+
+```cpp
+#define JSON_HAS_FILESYSTEM /* value */
+#define JSON_HAS_EXPERIMENTAL_FILESYSTEM /* value */
+```
+
+When compiling with C++17, the library provides conversions from and to
+[`std::filesystem::path`](https://en.cppreference.com/w/cpp/filesystem/path). As compiler support for filesystem is
+limited, the library tries to detect whether
+[`<filesystem>`/`std::filesystem`](https://en.cppreference.com/w/cpp/header/filesystem) (`JSON_HAS_FILESYSTEM`) or
+[`<experimental/filesystem>`/`std::experimental::filesystem`](https://en.cppreference.com/w/cpp/header/experimental/filesystem)
+(`JSON_HAS_EXPERIMENTAL_FILESYSTEM`) should be used. To override the built-in check, define `JSON_HAS_FILESYSTEM` or
+`JSON_HAS_EXPERIMENTAL_FILESYSTEM` to `1`.
+
+## Default definition
+
+The default value is detected based on the preprocessor macros `#!cpp __cpp_lib_filesystem`,
+`#!cpp __cpp_lib_experimental_filesystem`, `#!cpp __has_include(<filesystem>)`, or
+`#!cpp __has_include(<experimental/filesystem>)`.
+
+## Notes
+
+- Note that older compilers or older versions of libstd++ also require the library `stdc++fs` to be linked to for
+  filesystem support.
+- Both macros are undefined outside the library.
+
+## Version history
+
+- Added in version 3.10.5.