Virtual Zarr stores collection

One of the goals here is to provide ready-to-use virtual Zarr stores for commonly used data. It would be nice to create a collection of these stores within the repository, and allow contributors to submit PRs introducing newly created virtual Zarr stores.

Ideally, each virtual Zarr store should be reproducible: the code to create the reference file should be included as a single python file, and accompanied by some documentation, e.g. a README file specifying when, by whom, for what the store was created and any additional relevant information.

I'd imagine something like this:

```
virtual_stores/
└───anasurf23/
│ README.md
│ reference.py
│ references.json
└───fgsurf23/
│ README.md
│ reference.py
│ references.json
```

- [ ] create a new directory containing virtual zarr stores
- [ ] specify a naming convention for directories containing the individual virtual Zarr stores
- [ ] specify a template for the `reference.py` file
- [ ] specify a template for the README.md file
- [ ] include section in the repository's README to explain how these "submissions" work

Start by reviewing the repository README and the proposed virtual_stores layout. Define the directory naming convention and templates for reference.py and README.md, then add the submission guidance to the repository README and verify that the collection structure supports reproducible virtual Zarr stores.

Written by the indexing model from the issue text.