Update formatting for placeholder variables

[heitortsergent]

We should follow the guidance from the Grafana writers' toolkit on how to format placeholder variables across our documentation, both for OSS and for Grafana Cloud k6 docs.

For details on how placeholder formatting works: Placeholder variables, and fallback to Google's style guide if there are any questions.

[Tharanishwaran]

@heitortsergent can i work on this issue ?