[doc][enhancement] clarifying installation commands for simple user

For a regular user the installation instructions were not completely straightforward in my opinion and in particular the pip install git+https://gitlab.com/mlgenetics/dnadna.git was failing if you were already in your personal conda environment (it wasn't loading the packages required for dnadna).

We figured out that a good solution was to create a new python (3.7 !) environment with pip and to go the pip install git+... in this environment.

I now suggest 3 "complete" solutions (screenshot below) right at the beginning, and the details on conda come later.

Eventually this might be replaced by something like conda install dnadna in the future

Note:
<the conda install -c conda-forge msprime is added as a temporary fix (for people to be able to test dnadna in the common weeks) because both Jean and I tried on older mac laptops and the automatic installation of msprime through pip install git+... was failing (on the other hand it was working on our current laptops, not sure if its an issue of mac vs ubuntu or something else). I'll open a specific issue about this, its not urgent

Bonus: changes in dnadna goal description