gitlab-runner

Runner doc restructuring: Revamp the admin section index page

See merge request !5678



Merged-by: Jon Glassman <jglassman@gitlab.com>
Approved-by: Darren Eastman <deastman@gitlab.com>
Approved-by: Jon Glassman <jglassman@gitlab.com>
Co-authored-by: Roshni Sarangadharan <rsarangadharan@gitlab.com>