Decouple build from pages job in lektor.yml
We should decouple building the Lektor website with deploying to GitLab Pages. Doing this would grant each project control over if and how their project is deployed to GitLab Pages.
For example, some websites like the blog might need some special post-processing to fix links which are created absolute (start with
/) and don't work in the context of Pages which always deploys to a subdirectory (eg. '/blog/`)