Update ArcGIS Pro installation instructions to use conda-forge package

MGET is now available as a [conda-forge package](https://anaconda.org/conda-forge/mget3). The [installation instructions](https://mget.readthedocs.io/en/v3.0.3/WindowsWithArc.html) currently advise installing MGET with pip. We should update them to advise installing with conda or mamba.