Update Slack links

[BoHristova]

Description:

Slack workspace links in the Gardener documentation are inconsistent. There are multiple link targets:

• Gardener Project workspace:

documentation/website/documentation/contribute/_index.md

Line 103 in 6380d32

We use the [Gardener Project workspace](https://gardener-cloud.slack.com) for public communication related to the Gardener project.

• Contribute website page:

https://github.com/gardener/gardener/blob/52251889b4a8bc3b2ef3777124f2af1f1567e2f5/docs/proposals/README.md?plain=1#L41

• Slack invite to the workspace:

documentation/website/documentation/_index.md

Line 21 in 6380d32

- **Slack**: Join the Gardener community on [Slack](https://join.slack.com/t/gardener-cloud/shared_invite/zt-33c9daems-3oOorhnqOSnldZPWqGmIBw)

Once a target URL is agreed on (direct workspace URL vs. invite link), all documentation should be updated to use that consistently.