Fixed outdated link

What does this MR do and why?

This merge request tries to change a drawer link that's going to the wrong page.

• It currently links to: https://gitlab.com/help/ci/runners/index
• It should link to: https://docs.gitlab.com/runner/register/

The link in this area is incorrect--it's SaaS instead of self-managed. The link is hosted in the gitlab-runner project so it has to be an external link.

We remove the Rails injection here as well since the external link is added as a Vue app constant instead.

Related to: #368732 (closed)

Screenshots or screen recordings

Screen_Recording_2022-08-16_at_13.36.59

How to set up and validate locally

1. Go to CI/CD > Editor then click on the Help button on the editor. The Help drawer will appear.
2. Verify that the make sure your instance has runners available link goes to https://docs.gitlab.com/runner/register/

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.