docs: CONTRIBUTING.md dead link (PR merge blocker)

[agardnerIT]

PR checks are failing and blocked due to the markdown link check.

The file docs/CONTRIBUTING.md contains a dead link to the old https://keptn.sh/community/#slack.

[✖] https://keptn.sh/community/#slack → Status: 404

Fix

As the Keptn project now exclusively uses the Cloud Native slack (#keptn channel), this is the correct link to Slack:

https://cloud-native.slack.com/archives/C017GAX90GM

So in docs/CONTRIBUTING.md change https://keptn.sh/community/#slack to https://cloud-native.slack.com/archives/C017GAX90GM

[agardnerIT]

@JanaganSaravanan please drop a dummy comment here so I can assign this to you so that no-one else also works on it

[sahil110802]

@agardnerIT i want to work on this issue, please assign