Deploy Github pages to an environment

[nikosbosse]

As suggested by @seabbs in #316 it would be good to deploy gh pages to an environment to render the docs at epiforecasts.io/scoringutils

[nikosbosse]

Possibly related to #519, I think it would be a good idea to tackle this and come up with a clear workflow for

• which version is on CRAN
• which version is on main
• which version is on the develop branch
• what gets published when and how

[nikosbosse]

Closing this as a duplicate of #535