Add high-level and technical documentation about DataLad datasets

It would be helpful to add high-level documentation that describes the motivation and use cases for DataLad datasets that mirror the Dandisets, as well as technical documentation that describes how to use these datasets.

Relevant repositories:

1. https://github.com/dandi/dandisets
2. https://github.com/dandisets
3. https://github.com/dandi/backups2datalad