Add description of Euclid clusters notebook

[troyraen]

The Euclid clusters notebook is being moved to this repo to be released. This issue is to add an entry for it in the following two files:

• tutorials/euclid/euclid.md
• notebook_metadata.yml

Will require a one sentence description of the notebook (same in both files). This should merge at the same time as #282 so please coordinate.