Add one-sentence description for Euclid galaxy clusters tutorial

[jkrick]

Adds a one-sentence description for the Euclid galaxy clusters tutorial (euclid_clusters_tutorial.md, from bsipocz's ENH_euclid_clusters branch) to the two relevant files:

• notebook_metadata.yml — new entry with title "Galaxy Clusters" and description
• tutorials/euclid/euclid.md — new card linking to the tutorial

Description used: "Use Euclid Q1 data to detect and validate a galaxy cluster."

Closes #283

[troyraen]

LGTM, thanks. I'll hit "request changes" just so it doesn't get accidentally merged too early since it needs to be coordinated with #282.

[bsipocz]

I cherry-pick this one commit into the other branch, so we don't need to keep track of the PR.

[bsipocz]

This is now part of #284, closing the PR.

Thanks @jkrick!