Update Slack workspace links and add Discord integration

[tuhaihe]

This commit updates all community communication channels following the suspension of the previous apache-cloudberry.slack.com workspace.

Changes include:

• Update all Slack invitation links to new asf-cloudberry workspace
• Add Discord server integration across the site
• Update channel lists for both Slack and Discord guides
• Add blog post announcing the communication channels update
• Update related documentation and support pages

closes: #ISSUE_Number

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[tuhaihe]

Hi @leborchuk PTAL. Thanks!

[leborchuk]

Checked Slack and Discord URL - both are valid. LGTM