set up GHA workflow for previewing PRs

https://github.com/rossjrw/pr-preview-action can be used to preview github pages deployments before they're merged to main. this would make things like dependabot PR review much easier as you might not need to check out any code - could just open the preview and see if it still works (assuming all other CI has passed!)

There may be an official option being developed: https://github.com/actions/deploy-pages/blob/main/action.yml#L29 but in the meantime it looks like we can use the above action.

obviously this needs to not break the main deployment!