[documentation] Conda installation doc

As mentioned, the current docs specifies the following installation:

# Download conda
wget https://repo.anaconda.com/miniconda/Miniconda3-py39_4.12.0-Linux-x86_64.sh
# Launch it
chmod +x ./Miniconda3-py39_4.12.0-Linux-x86_64.sh ; ./Miniconda3-py39_4.12.0-Linux-x86_64.sh
# Download git
apt install git -y -q
# Download autosubmit
git clone https://earth.bsc.es/gitlab/es/autosubmit.git -b v4.0.0b
cd autosubmit
# Create a Conda environment
conda env update -f environment.yml -n autosubmit python=3.7
# Activate env
conda activate autosubmit
# Test autosubmit
autosubmit -v

Of course, it is wrong as conda env update don't need the python version as is already on the YAML. Then, it needs autosubmit to be installed afterwards because enviroment.yml only specifies the dependencies.