Add mempool.space instance to doppler cluster

[tee8z]

To help with development allow for user to include a mempool.space explorer instance into the cluster, so they can view their transactions as they move through.

[tee8z]

Looking at this more deeply, I'm hesitant to add this as a supported tool due to how many additional images would need to be added into the cluster for it to run:
https://github.com/mempool/mempool/blob/master/docker/docker-compose.yml

It requires frontend, backend, and DB images. I will hold off on adding this as a supported tool until there is an actual demand for this option.