Document last_used_at and usage_type in the deploy keys API

What does this MR do and why?

Every deploy key response carries last_used_at and usage_type. API::Entities::DeployKey inherits both from API::Entities::SSHKey, where they are exposed with no condition, so they are on every response of every endpoint on this page. last_used_at appeared nowhere on the page and usage_type appeared in exactly one of the nine example responses.

This adds both to all nine example responses, which is twelve deploy key objects in total. I put last_used_at next to expires_at, where the entity exposes it, and usage_type next to key, for the same reason. There are no response attribute tables on this page, only request parameter tables, so nothing else needed updating.

While editing the examples I also fixed three keys that had lost their name and were left as "", all of them fingerprint_sha256: two in "List all deploy keys" and one in "List project deploy keys for user". They parse as valid JSON, so nothing complains, but anyone copying the example gets an object with an empty key and no SHA256 fingerprint.

Values: usage_type is auth in the examples I added, matching the entity's own documented example, and the one that already showed auth_and_signing is untouched. last_used_at is a timestamp after each key's created_at.

The two project access lists are correct as they stand and I left them alone. They are exposed under if: ->(_, options) { options[:include_projects_with_write_access] } and its readonly twin, and GET /deploy_keys is the only route that passes those options, which is why they appear in the instance list example and in none of the project ones.

I found this while adding the same two fields to the Go API client, in gitlab-org/api/client-go!3049 (merged).

MR acceptance checklist

This change is documentation only. No endpoint, parameter or behaviour changes. I parsed every JSON block on the page after editing: all nine still parse, and all twelve deploy key objects now carry both fields.