[Task](ui-docs): migrate deployment to github pages so this secret concourse/teams/services/github-private-key/ui-docs can be removed

[ArtieReus]

Task Description
Migrate the deployment of our internal team documentation to GitHub Pages, allowing us to remove the secret concourse/teams/services/github-private-key/ui-docs. As far as I remember, we initially chose the current approach because GitHub Actions wasn’t ready for internal deployments. Now that it is, we can switch and save some resources.