Add workflow_dispatch to enable us to manually rebuild the providers-dashboard pages

[rartino]

Github actions has a feature where one can enable a workflow trigger:

https://github.blog/changelog/2020-07-06-github-actions-manual-triggers-with-workflow_dispatch/

This makes a button appear under "Actions" in GitHub that when clicked trigger the workflow. This would be a nice feature to have for the providers-dashboard so that there is a way to manually re-trigger generation of the pages without having to wait for the daily re-generation.