Add Fleet & Agent 8.16.0 Release Notes

[kilfoyle]

This adds the 8.16.0 Fleet & Elastic Agent Release Notes:

• Fleet contents from Kibana Release Notes PR
• Fleet Server contents from BC3 changelog
• Elastic Agent contents from BC3 changelog

See DOCS PREVIEW

Closes: #1411

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[kilfoyle]

@cmacknz As you suggested in the feature summary issue, I've added in descriptions from that issue for the release features, in commit 9323922.

The "installing agent using Helm" feature wasn't in the release notes originally, I think because the PR has a skip-changelog tag. I've added that PR into the features section but let me know if it shouldn't be there for any reason.

[cmacknz]

The Helm chart being GA should definitely be in here, thanks

[kilfoyle]

Thanks @kpollich. I've added in all of your suggestions.

[kilfoyle]

I've added a breaking change for Elastic Agent (Beats 39993) as suggested by Luca here.

[kpollich]

LGTM 🚀