Dimension's setup is unnecessarily complex

The procedure to [configure Dimension](https://github.com/Automattic/matrix-env#configuring-dimension) is unnecessarily complex, due to the fact that the `dimension` user's `access_token` must be set under `dimension/config.yaml`.

This creates several problems:

1. The setup instructions are more complex since we can't fully configure dimension out of the box
2. The `dimension/config.yaml` file will constantly be changed in the local checkout of this repository
3. The access token is committed into the repository

These issues would be fixed if dimension allowed setting the access token through an environment variable.

Start with the linked Dimension configuration procedure and the dimension/config.yaml file to understand how the access_token is currently supplied. Check whether Dimension supports environment-based token configuration; done means setup no longer requires committing or repeatedly changing the token in the local checkout.

Written by the indexing model from the issue text.