Update Slack links to include both invite and direct channel URLs

[Copilot]

(I've edited Copilot's PR description as I found it way to verbose)

I found a couple of useful links can be found via the project icons at https://contribute.cncf.io/contributors/projects/:

#cert-manager: https://kubernetes.slack.com/messages/cert-manager #cert-manager-dev: https://kubernetes.slack.com/messages/cert-manager-dev

I propose to add these links so that folks can directly jump to the correct channel once they joined the invite link (https://slack.k8s.io/).

CyberArk tracker: VC-47746