Use only internal references inside docs/

Author: mathbunnyru

docs/contributing/tests.md

@@ -14,7 +14,7 @@ More info on `pytest` can be found [here](https://docs.pytest.org/en/latest/cont
 The actual image-specific test files are located in folders like `tests/by_image/<somestack>/` (e.g., `tests/by_image/docker-stacks-foundation/`, etc.).
 
 ```{note}
-If your test is located in `tests/by_image/<somestack>/`, it will be run against the `jupyter/<somestack>` image and against all the [images inherited from this image](https://jupyter-docker-stacks.readthedocs.io/en/latest/using/selecting.html#image-relationships).
+If your test is located in `tests/by_image/<somestack>/`, it will be run against the `jupyter/<somestack>` image and against all the [images inherited from this image](../using/selecting.md#image-relationships).
 ```
 
 Many tests make use of global [pytest fixtures](https://docs.pytest.org/en/latest/reference/fixtures.html)

docs/using/custom-images.md

@@ -4,7 +4,7 @@ This section describes how to build a custom set of images.
 It may be helpful if you need to change the Ubuntu or Python version, or to make a significant change to the build process itself.
 
 This project only builds one set of images at a time.
-If you want to use older images, take a look [here](https://jupyter-docker-stacks.readthedocs.io/en/latest/#using-old-images).
+If you want to use older images, take a look [here](../index.rst/#using-old-images).
 
 ## Automating your build using template cookiecutter project