Add example email on push notification to docs

What does this MR do?

Adds example screenshot of the notification, and replaces original screenshot with current design and smaller size (KB).

Related issues

Relates to https://gitlab.com/gitlab-org/gitlab-ce/issues/54831 – didn't know what the notification looked like.

Author's checklist

• Apply the correct labels and milestone
• Crosslink the document from the higher-level index
• Crosslink the document from other subject-related docs
• Feature moving tiers? Make sure the change is also reflected in features.yml
• Correctly apply the product badges and tiers
• Port the MR to EE (or backport from CE): always recommended, required when the ee-compat-check job fails

Review checklist

• Your team's review (required)
• PM's review (recommended, but not a blocker)
• Technical writer's review (required)
• Merge the EE-MR first, CE-MR afterwards