Add introduction to shared runners page
Problem to solve
Shared Runners are an important benefit GitLab offers and they help people get up and running much more quickly than you can by setting them up yourself. We should add a page that welcomes people to them, describes how to use them, and in an easy to read way for new users.
We should also help redirect people who are ended up at the Runner setup page who might be confused.
This impacts the Runner documentation but should also be linked to from the CI new user area.
Further details
This is mainly intended for new users trying GitLab. The goal is to help get them up and running quickly with CI.