2015 improve explanation of datalist format (#2019)

Fixes #2015  .

### Description
Added explanation of datalist format for easier quick-start for new
users interested in using the Auto3DSeg AutoRunner. Also simplified the
datalist explanation in `run_with_minimal_input.md`, and specified that
the `msd_datalist_generator.ipynb` notebook is only used to generate a
datalist with cross-validation folds from an existing MSD dataset (and
renamed it accordingly to `msd_crossval_datalist_generator.ipynb`).

New users landing on the main README of the Auto3DSeg tutorial will now
directly see how to create a datalist for their own data, and can easily
find where further steps are described.

I also removed a paragraph on epoch/iteration numbers from
the`auto_runner.ipynb` that is no longer relevant since MONAI v1.1.

There is a corresponding PR
[here](https://github.com/Project-MONAI/MONAI/pull/8539) that contains
improvements to the MONAI library with regards to the datalist format.

### Checks
<!--- Put an `x` in all the boxes that apply, and remove the not
applicable items -->
- [x] Avoid including large-size files in the PR.
- [ ] Clean up long text outputs from code cells in the notebook.
- [ ] For security purposes, please check the contents and remove any
sensitive info such as user names and private key.
- [ ] Ensure (1) hyperlinks and markdown anchors are working (2) use
relative paths for tutorial repo files (3) put figure and graphs in the
`./figure` folder
- [ ] Notebook runs automatically `./runner.sh -t <path to .ipynb file>`

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Signed-off-by: Daniël Nobbe <daniellnobbe@gmail.com>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Eric Kerfoot <17726042+ericspod@users.noreply.github.com>