Keep slack.k8s.io invite link alongside direct channel links

Signed-off-by: Maël Valais <[email protected]>

Author: Copilot

content/docs/releases/README.md

@@ -119,8 +119,8 @@ Only the last patch release of each branch is supported.
 ### Technical support
 
 Technical assistance is offered on a best-effort basis for supported
-releases only. You can request support from the community on Kubernetes
-Slack (in the [`#cert-manager`](https://kubernetes.slack.com/messages/cert-manager) or [`#cert-manager-dev`](https://kubernetes.slack.com/messages/cert-manager-dev) channels), using
+releases only. You can request support from the community on [Kubernetes
+Slack](https://slack.k8s.io/) (in the [`#cert-manager`](https://kubernetes.slack.com/messages/cert-manager) or [`#cert-manager-dev`](https://kubernetes.slack.com/messages/cert-manager-dev) channels), using
 [GitHub Discussions][discussions] or using the [cert-manager-dev][group]
 Google group.
 

content/v1.15-docs/releases/README.md

@@ -108,8 +108,8 @@ Only the last patch release of each branch is supported.
 ### Technical support
 
 Technical assistance is offered on a best-effort basis for supported
-releases only. You can request support from the community on Kubernetes
-Slack (in the [`#cert-manager`](https://kubernetes.slack.com/messages/cert-manager) or [`#cert-manager-dev`](https://kubernetes.slack.com/messages/cert-manager-dev) channels), using
+releases only. You can request support from the community on [Kubernetes
+Slack](https://slack.k8s.io/) (in the [`#cert-manager`](https://kubernetes.slack.com/messages/cert-manager) or [`#cert-manager-dev`](https://kubernetes.slack.com/messages/cert-manager-dev) channels), using
 [GitHub Discussions][discussions] or using the [cert-manager-dev][group]
 Google group.