Update screenshots regularly

Summary

As mentioned by @ghickman on Slack, we could use something like shot-scraper to generate screenshots.

This ideally would be automated or, failing that, semi-automated and run by a developer locally.

Benefits

Inspired by https://twitter.com/aaronbassett/status/1536708194160070658:

• For users, this keeps screenshots up-to-date with the current appearance of web services.
• For developers, these act as simple additional checks, warning us when web services have changed substantially, either when:
  • the layout has changed somewhat, and a new screenshot does not represent what was intended
  • the layout has changed enough that this automation fails

Questions/ideas

• What's the best way of automating login to external services where necessary?
  • Relevant issue for shot-scraper: https://github.com/simonw/shot-scraper/issues/30
  • Can we configure 2FA for such services to secure test accounts? This seems a little tricky.
• Would some services block GitHub Actions runners? I've seen this when automating link checks.
• For services that we develop, we should probably run them with a GitHub Actions runner to screenshot, instead of trying to login into the production site.
• What about screenshots of, for example, Docker or GitHub Desktop installation processes?
• Do we want to image diff and use that in automation, to determine when to update?

Start by evaluating the proposed shot-scraper approach and the GitHub Actions automation questions described in the issue, including authentication, runner blocking, and image diffs. Done is a decided, repeatable approach for generating and updating service screenshots, with the automation scope and handling of external services documented.