Sidetrail readme contains invalid instructions for running tests locally

[goatgoose]

Problem:

The sidetrail readme contains instructions for running the sidetrail tests locally. These instructions indicate that a sidetrail Dockerfile should be used to build an image. However, this Dockerfile was removed in #3518, so it no longer exists.

Solution:

We should remove the invalid reference from the readme, and instead link to the instillation scripts to setup the dependencies locally without docker.

After #4479 is resolved, we can link to the sidetrail docker image again.

[maddeleine]

Hah, apparently someone has noticed this before: #3881