Docs: add discord integration docs

[fern-support]

Overview

This PR adds documentation for discord integration:

• Replaces: Instructions sending users to support for discord setup with references to the new discord doc.
• Adds: References to the new discord doc wherever there are references to slack

Preview to the new Discord doc

Preview to the Overview page with added discord Card

[github-actions]

🌿 Preview your docs: https://fern-preview-73b8aeb5-a7ce-447e-a5e9-fee1b5fd5f8e.docs.buildwithfern.com/learn

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