Add zarr merger notebook section (#1433)

This PR add an example for `ZarrAvgMerger` to be plugged into
`PatchInferer` along with `WSISlidingWindowSplitter`.

Note: Project-MONAI/MONAI#6633 in MONAI core
should be merged before this one.

### 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.
- [x] Clean up long text outputs from code cells in the notebook.
- [x] For security purposes, please check the contents and remove any
sensitive info such as user names and private key.
- [x] 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
- [x] Notebook runs automatically `./runner.sh -t <path to .ipynb file>`

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Signed-off-by: Behrooz <[email protected]>