Document expanded panel feature and contextual menu

Goal

We should document the single panel feature:

• a new screenshot for https://docs.gitlab.com/ee/user/project/integrations/prometheus.html#chart-context-menu
• Adding the full-screen option to the list of links, and explaining how to get out of the full-screen option.

Original Discussion

The following discussion from !30126 (merged) should be addressed:

• @mrincon started a discussion: (+1 comment)

  @aqualls Do you think extra documentation is needed here? I tend to think it is not needed.