Add CI step to publish Evidence reports to GitHub pages

[aaronsteers]

The reports are working well on my local workstation, but I want to also add a BI publish step into the repo. We could use Netlify or similar, but my preference would be to use GitHub Pages, if possible, specifically so that people who fork the repo or use it as a template will not be required to create an account on another service.

If we get this working well, we could then see about also getting dbt Docs published in the same way.

cc @edgarrmondragon, @archiewood