Update secrets keyword to new YAML reference style
As part of &6941 (closed), we need to update the secrets
keyword to align with the style defined at https://docs.gitlab.com/ee/development/cicd/cicd_reference_documentation_guide.html.
For more examples, see the already completed MRs listed in the epic.
The goal is to make this page a purely reference page, with all concept and task details on more appropriate pages related/dedicated to each feature.
Edited by Marcel Amirault