Update docs w.r.t. installation

[uwefladrich]

We should be updating the documentation now after the conda-forge packages have been released (#92). The "installation using conda" part can probably be collapsed to a conda install scriptengine-tasks-ecearth but we need to rethink the pip part and possibly other details. Would be good to give this section a general fresh-up in order to make sure everything is clear, consistent and as easy as possible.

[valentinaschueller]

closed by #97