add guide or section on how to use cert-manager driven certs in Kuma

[lahabana]

Description

With kumahq/kuma#9342 it becomes easy to use cert-manager for cp certs.

It would be nice to have a guide for it:

cert manager stuff used:

apiVersion: cert-manager.io/v1
kind: ClusterIssuer
metadata:
  name: selfsigned-issuer
spec:
  selfSigned: {}
---
apiVersion: cert-manager.io/v1
kind: Certificate
metadata:
  name: kuma-selfsigned-ca
  namespace: kuma-system
spec:
  isCA: true
  commonName: kuma-selfsigned-ca
  secretName: root-secret
  privateKey:
    algorithm: ECDSA
    size: 256
  issuerRef:
    name: selfsigned-issuer
    kind: ClusterIssuer
    group: cert-manager.io
---
apiVersion: cert-manager.io/v1
kind: Issuer
metadata:
  name: kuma-issuer
  namespace: kuma-system
spec:
  ca:
    secretName: root-secret
---
apiVersion: cert-manager.io/v1
kind: Certificate
metadata:
  name: control-plane-cert
  namespace: kuma-system
spec:
  secretName: control-plane-cert
  duration: 2160h # 90d
  renewBefore: 360h # 15d
  isCA: false
  privateKey:
    algorithm: RSA
    encoding: PKCS1
    size: 2048
  usages:
    - server auth
  dnsNames:
    - kuma-control-plane.kuma-system.svc
    - kuma-control-plane
    - kuma-control-plane.kuma-system
    - kuma-control-plane.kuma-system.svc.local
  issuerRef:
    name: kuma-issuer
    kind: Issuer

values:

controlPlane:
  tls:
    general:
      secretName: control-plane-cert

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