Fix Gitlab pages and proposal of new branching model

This MR fixes broken update on https://maicos-devel.gitlab.io/maicos/ when we release a new version. The reason is that gitlab requires a job name with the name pages. For any other name the docs will not be deployed.

Additionally, I propose that we deploy the docs and the code on pypi only when we create a tag. We always create a tag when we release a new version. This makes the two branches master and develop a bit superfluous and we can go (back) to a single branch model named main. What do you think?

I got the idea of the single branch model since mda is now also going back to single branch...