Fix broken Slack invitation links and centralize URL management

[Copilot]

Fix Broken Slack Links on Community Page

This PR addresses the issue where the Slack link on the /community page doesn't work anymore, showing "This link is no longer active" message.

Changes made:

• Analyze the repository structure and locate Slack link references
• Identify the correct Slack invitation URL format
• Update all user-facing Slack links to use https://valkey.io/slack
• Remove unused config variable
• Test the fix by building the site
• Verify the links work correctly through the redirect

Root cause:
The community page and blog post were using malformed Slack URLs that included invalid fragments.

Centralized approach:
All user-facing Slack links now use https://valkey.io/slack which provides a consistent, maintainable URL that redirects to the current Slack invitation. The redirect is handled by content/slack.md which contains the actual invitation URL.

Files changed:

• content/community.md - Updated to use https://valkey.io/slack
• content/blog/2025-08-04-valkey-swift/index.md - Updated to use https://valkey.io/slack
• config.toml - Removed unused configuration variable

This approach ensures all Slack links are consistent and maintainable, with only the redirect page needing updates when the invitation URL changes.

Fixes #365.

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[stockholmux]

LGTM - I tested and it seems to work but I suspect Slack is doing some hocus pocus that might be making this non-deterministic.

At least this is a step in the right direction though.