docs: remove references to slack

[backflip]

As our Slack team is not really active anymore I would recommend the following:

• Removing references to it and suggesting issues as main way of communicating.
• Adding a link to the lively Accessibility Club Slack team instead.
• Closing the Slack team.

What does this change achieve:

• We can lead people to an active community where they can post any question about accessibility and get a quick answer.
• Questions about the ADG itself are answered here instead of here and in Slack.
• We don't have to maintain a Slack team.

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[backflip]

@christophdubach, good question, I have no idea. Was this happening via Slack so far?

[christophdubach]

I just checked: Since there is no dedicated channel for this in our Slack, probably not. 😅