Reorganize directories (#3462)

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

docs/mkdocs/docs/integration/pkg-config.md

@@ -0,0 +1,13 @@
+# Pkg-config
+
+If you are using bare Makefiles, you can use `pkg-config` to generate the include flags that point to where the library is installed:
+
+```sh
+pkg-config nlohmann_json --cflags
+```
+
+Users of the [Meson build system](package_managers.md#meson) will also be able to use a system-wide library, which will be found by `pkg-config`:
+
+```meson
+json = dependency('nlohmann_json', required: true)
+```