feat: Revamp Slack Onboarding Page with Enhanced Visuals and Content (#…

…389)

### What
- **Repository Addition:** Add the Slack Onboarding page to the
  repository for better version control and collaboration. Note:
  Deployment still requires manual transfer to `/var/www/slack/index.html`
  on `proxi1.openfoodfacts.org`.
- **Content and Visual Update:** The page has been significantly
  improved with more engaging text and eye-catching illustrations to
  increase community conversion rates. These enhancements aim to make the
  onboarding experience more inviting for new users. The updated content
  remains monolingual.

Fix #388