fix: update beta alert to remove forum link and update Slack URL

[evwilkin]

Summary

Fixes #4900

This PR updates the beta component alert message in the MDX template to:

• Remove the link to the PatternFly forum (which has been sunsetted)
• Update the Slack URL from //slack.patternfly.org/ to https://patternfly.slack.com

Changes

• File modified: packages/documentation-framework/templates/mdx.js
• Removed forum reference from beta alert message (lines 79-80)
• Updated Slack URL to use proper HTTPS format

Test plan

• Verify beta alert displays correctly on beta component documentation pages
• Confirm Slack link opens correctly to https://patternfly.slack.com
• Ensure no forum link appears in the beta alert message

🤖 Generated with Claude Code

[patternfly-build]

Preview: https://pf-org--pr-4903-site.surge.sh