Add version history for SSH key usage types

What does this MR do?

This merge request is a first attempt to catch up with the changes of #383046 (closed), which went out in !106176 (merged) with no docs. (@tlinz, this merge request is a good example of why I need to know about feature flag merge requests, but in fairness, it's VERY easy to miss something in the December milestone because so many people are out.)

This MR has two problems:

1. Blurred lines between groups. The feature described here blurs the lines between groupsource code and ~"group::authentication and authorization". The engineering work was done by Source Code, but the page belongs to ~"group::authentication and authorization". The ssh_key_usage_types feature flag came and went within a single milestone, so I think we're safe in not documenting it. However, now that it's gone, we should consider what other changes doc/user/ssh.md needs. We likely need to talk about this new scope in more detail, how it can be used, and what it requires, but …
2. Amy is still not fully caught up from sick leave. I'm still sifting through what happened during vacation-followed-by-Covid. I promised @dianalogan in last week's 1:1 that if I could hand off any work, I would. I don't know if @jglassman1 has bandwidth to pick up the rest of the work, and I'm hesitant to ask, but I don't know this docset well.

What I'll do here is assign this small MR review over to Jon. I intentionally kept it small, so I could also use it as a discussion point for finishing up the rest of the feature work.

Related issues

• Related to [Feature flag] Rollout of `ssh_key_usage_types` (#383046 - closed)
• Related to [Feature flag] Cleanup `ssh_commit_signatures` (#384202 - closed)
• Related to Cleans up `ssh commit signatures` ~"feature flag" (!106176 - merged) where the flag changes got merged sans docs

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