Fix broken Slack invitation links and centralize URL management (#366)

## 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page doesn't work anymore, showing "This link is no longer active"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.

![Updated Community
Page](https://github.com/user-attachments/assets/e39a5f09-0c78-48e8-93ac-f4e76bc43145)

**Changes made:**
- [x] Analyze the repository structure and locate Slack link references
- [x] Identify the correct Slack invitation URL format  
- [x] Update all user-facing Slack links to use
`https://valkey.io/slack`
- [x] Remove unused config variable
- [x] Test the fix by building the site
- [x] 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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Author: Copilot

content/blog/2025-08-04-valkey-swift/index.md

@@ -199,4 +199,4 @@ We'd love you to try the client out and get your feedback.
 
 ### Connect with us
 
-If you want to discuss any of the above, want to report a bug or want to contribute please reach out to us on [GitHub](https://github.com/valkey-io/valkey-swift/) or talk to us in the `#valkey-swift` channel on the [Valkey Slack](https://valkey-oss-developer.slack.com/join/shared_invite/zt-2nxs51chx-EB9hu9Qdch3GMfRcztTSkQ#/shared-invite/email).
+If you want to discuss any of the above, want to report a bug or want to contribute please reach out to us on [GitHub](https://github.com/valkey-io/valkey-swift/) or talk to us in the `#valkey-swift` channel on the [Valkey Slack](https://valkey.io/slack).

content/community.md

@@ -7,7 +7,7 @@ template = "community.html"
 cards = [
   { icon = "icon-question.svg", title = "Ask Questions", description = "If you have any inquiries about Valkey, feel free to join the conversation on our GitHub discussions or chat with us on Slack.", links = [
     { url = "https://github.com/orgs/valkey-io/discussions", text = "GitHub Repository" },
-    { url = "https://valkey-oss-developer.slack.com/join/shared_invite/zt-2nxs51chx-EB9hu9Qdch3GMfRcztTSkQ#/shared-invite/email", text = "Slack" }
+    { url = "https://valkey.io/slack", text = "Slack" }
   ] },
   { icon = "icon-bug.svg", title = "Report Bugs", description = "If you encounter any issues while using Valkey, please help us improve the project by filing a bug report at our GitHub repository.", links = [
     { url = "https://github.com/valkey-io/valkey/issues/new?assignees=&labels=&projects=&template=bug_report.md&title=%5BBUG%5D", text = "GitHub Repository" }