Update group runner registration instructions

What does this MR do and why?

Update the Group Runner registration instruction to reflect new workflow after implementing New Group Runners view MVC

Current instructions:

To create a group runner:

1. Install GitLab Runner.
2. On the top bar, select Main menu > Groups and find your group.
3. On the left sidebar, select CI/CD > Runners.
4. Note the URL and token.
5. [Register the runner](https://docs.gitlab.com/runner/register/

don't reflect the current view

as the URL and token are not displayed in the Group Runners view.

Related issues

#19819 (closed)

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Closes #377352 (closed)