Remove the D2 Slack integration docs

[alixander]

What changed

• Removed the Slack tour page from the D2 documentation.
• Removed Slack from the extensions overview and navigation.
• Kept the historical 0.1.3 release note factual without linking to an installer that no longer exists.

Why

The D2 Slack integration is no longer supported, and the old install flow cannot complete. The documentation should not expose a paid-account requirement or a broken OAuth button.

User impact

The Slack integration is no longer advertised or installable from d2lang.com. No wind-down messaging is added.

Validation

• Docusaurus production build completed for English and Korean.
• Confirmed the build no longer generates /tour/slack/.
• Confirmed no active Slack install URL remains in the docs or sidebar.
• git diff --check passed.