Update ANTS installation instructions to use `-n` for conda env

# 📚 Documentation Request

## Addition
An end user has reported that the use of `-p` to specify a full path for a conda environment in our [​installation instructions](https://metoffice.github.io/ANTS/install.html) means that the environment does not have a name that is shown with `conda env list` or can be used with `conda activate`. Instead the full path is needed.

We should amend our installation instructions to use the `-n` argument.

Start with the ANTS installation instructions at https://metoffice.github.io/ANTS/install.html and locate the conda environment creation command using `-p`. Change the documented command to use `-n`, then verify the rendered instructions show a named environment usable with `conda env list` and `conda activate`.

Written by the indexing model from the issue text.