Update index.md to include documentation in for the `--access-level` param values
What does this MR do?
This MR adds clarification for the GitLab Runner parameter --access-level
within the Runner registration one-liner. This param was added in 12.0 to allow a Runner to be registered as a Protected Runner upon registration. The documentation failed to explain this change or list acceptable values for the --access-level
param.
Related MR talking about acceptable values for this param: !1323 (merged)