Update instructions on Mamba installation #318

The instructions for installing mamba as part of the ESMValTool installation in Installation may need to be modified as there is a recommendation to use Miniforge vas Mambafogre now. I have created an issue #3597 under ESMValTool.

We should follow whatever the technical team suggests and changes this to in the main documentation. For now, I have left the instructions as is for the Installation episode.

-Ranjini