executors: Improve documentation of binary installation

Created by: eseliger

Feedback from customer:

with the currently documented approach for downloading the executor binary, its not obvious if I need to update the version of the executor binary that I'm currently using. Since the feature is now in beta, I think it might be a good idea to introduce formal versioning (a separate changelog ) for the executors binary or document if the expectation is that executors should be updated every time there is a new release of Sourcegraph.