Combine both Automated Rotation of TLS Credentials documents for control-plane and webhooks

Nobody has claimed this yet.

Currently there are some differences in the two documents that make it hard to achieve both.

[Automatically rotating control plane TLS credentials](https://linkerd.io/2-edge/tasks/automatically-rotating-control-plane-tls-credentials/)

- In this document works fine

[Automatically rotating webhook TLS credentials](https://linkerd.io/2-edge/tasks/automatically-rotating-webhook-tls-credentials/)

- This document assumes you have NOT setup rotating control plane TLS credentials yet and it can make it confusing to understand.
- Also there is no mention of using cert manager to make the issuer or the alternative clusterIssuer like in the rotating control plane TLS credentials document.

[See my comment here explaining how to use cert manager to issue webhook certs](https://github.com/linkerd/linkerd2/discussions/8575#discussioncomment-14972528)

I could be doing something dumb with my solution and would appriciate any advice!

Maybe also add details on how the expiration and rotation steps flow.

Start by comparing the linked control-plane and webhook TLS credential documents, then review the referenced discussion comment about using cert-manager. Consolidate the overlapping guidance so the combined document covers issuer and ClusterIssuer options, works when control-plane rotation is already configured, and explains the expiration and rotation flow.

Written by the indexing model from the issue text.