Add a preview environment for pull requests

[mschoettle]

It would be great to have a preview environment (on demand) for a PR in order to be able to look at the rendered site directly without having to do it manually.

Some references:

• GitHub Documentation workflow: https://github.com/github/docs/blob/08fb998082f9ef7090fdde26d4e00673a1eaac7e/.github/workflows/azure-preview-env-deploy-public.yml (has since been deleted)
• https://github.com/marketplace/actions/preview-environments
• https://medium.com/@sorindumitrescu/create-preview-environments-from-your-github-pipeline-f9bbcef17fc0