Added ssh host key fingerprints to docs/computing/connecting.md

[lupreCSC]

Proposed changes

Added ssh host key fingerprints to docs/computing/connecting.md. These were previously buried in the What's new > Computing environment section and didn't show up in the search. Since users should verify these when prompted by SSH upon first connection (and when the key changes), it makes sense to include them on the page about Connecting.

Also added a brief note about needing to check the key and confirm the host upon first connection, which was a problem for a user in a recent RT ticket. preview page

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