Improve document for DNS-01 Challenge with CNAME Delegation

[MartinWeindel]

How to categorize this issue?

/area documentation
/kind task

What would you like to be added:
The Follow CNAME feature is documented only in the cert-management, but not in the shoot-cert-service itself.

Compare with https://github.com/gardener/cert-management?tab=readme-ov-file#follow-cname

Why is this needed:

[philsbln]

I totally second that.

Maybe also add a hint that pointing to "_acme-challenge.gardener.target.domain" is a bad idea…

I currently have

webres6.dev.sap.                 3600 IN CNAME  prod.webres.v6only.shoot.canary.k8s-hana.ondemand.com.
_acme-challenge.webres6.dev.sap. 3600 IN CNAME  _acme-challenge.prod.webres.v6only.shoot.canary.k8s-hana.ondemand.com.

which works but also blocks getting a cert for prod.webres.v6only.shoot.canary.k8s-hana.ondemand.com

[gardener-ci-robot]

The Gardener project currently lacks enough active contributors to adequately respond to all issues.
This bot triages issues according to the following rules:

• After 90d of inactivity, lifecycle/stale is applied
• After 60d of inactivity since lifecycle/stale was applied, lifecycle/rotten is applied
• After 60d of inactivity since lifecycle/rotten was applied, the issue is closed

You can:

• Mark this issue as active with /lifecycle active
• Mark this issue as frozen with /lifecycle frozen
• Mark this issue as fresh with /remove-lifecycle stale
• Mark this issue as rotten with /lifecycle rotten
• Close this issue with /close

/lifecycle stale

[MartinWeindel]

/lifecycle frozen