Use meaningful link text in Autoscaling GitLab Runner doc

Note: The relevant doc is now in the Runner project

https://gitlab.com/gitlab-org/gitlab-runner/blob/master/docs/configuration/runner_autoscale_aws/index.md
->
https://docs.gitlab.com/runner/configuration/runner_autoscale_aws/

Update this doc to rephrase all instances of link text "read more" to a descriptive name for the destination of the link.

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This is a follow-up from "Add article on autoscaling GitLab Runner on AWS"

The following discussion from gitlab-ce!15313 should be addressed:

• @marcia started a discussion:

  @axil since you're keeping the "read more” lines, let's at least make the links meaningful. http://www.agconsult.com/en/usability-blog/meaningful-links-are-a-must

  So, instead of linking from [read more](), let’s link from [[runners.machine]]().

  😉

  But I don’t see this as a reason for delaying the article, so I’ll merge it and you can do it afterward, if that’s ok.