Update documentation

Update the installation and usage documentation to reflect changes made in recent merges, notably the refactoring of the datamodules and the addition of saliency maps.

doc/usage/aggregpred.rst

-.. Copyright © 2023 Idiap Research Institute <contact@idiap.ch>
-..
-.. SPDX-License-Identifier: GPL-3.0-or-later
-
-.. _mednet.usage.aggregpred:
-
-=======================================================
- Aggregate multiple prediction files into a single one
-=======================================================
-
-This guide explains how to aggregate multiple prediction files into a single
-one. It can be used when doing cross-validation to aggregate the predictions of
-k different models before evaluating the aggregated predictions. We input
-multiple prediction files (CSV files) and output a single one.
-
-Use the sub-command :ref:`aggregpred <mednet.cli>` aggregate your prediction
-files together:
-
-.. code:: sh
-
-   mednet aggregpred -vv path/to/fold0/predictions.csv path/to/fold1/predictions.csv --output-folder=aggregpred
-
-
-.. include:: ../links.rst