Add release notes for v1.16.0-alpha.0

[wallrj]

Preview: https://deploy-preview-1526--cert-manager.netlify.app/docs/releases/release-notes/release-notes-1.16#changes-since-v1150

Part of the v1.16.0-alpha.0 release process.

• https://kubernetes.slack.com/archives/CDEQJ0Q8M/p1721836875640659

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[maelvls]

nit, don't feel the need to change anything: I wish we could write release notes that are slightly more user-focused, but I realize that it isn't that easy since each contributor writes their own release notes. Among others:

• Most of these items focus on what was technically done to achieve the feature (e.g., "add X" rather than explaining whether/how it can impact the user.
• "Features" is super heterogenous; I think the Helm improvements should be in their own section or subsection, and maybe also group "Runtime Improvements", and "API Changes".

Example:

• Add SecretRef support for Venafi TPP issuer CA Bundle

should be reworded as:

• The Venafi issuer now lets you load your custom CA bundle to connect to TPP from a Secret resource using the new field secretRef.

I think we would benefit greatly from having a technical writter look at our release notes from time to time. Maybe we could get @ditatechwriter (Michael) to give some advice on this.

[maelvls]

Since this is an alpha, I don't think it's super important to write great release notes. But for the final release, we surely do! I've left a comment in that sense.

/lgtm
/approve

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