Add contributor recognition and rewards information for translators

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What does this MR do and why?

Add contributor recognition and rewards information for translators

Closes #558242 (closed)

This MR adds information about GitLab's contributor recognition and rewards system for translators contributing through Crowdin. The information was previously only available through Crowdin discussions, making it difficult for translators to discover and participate in the recognition system.

Changes included:

• Add new "Contributor Recognition and Rewards" section to the translation documentation
• Include instructions for linking Crowdin and GitLab accounts
• Provide information about leaderboard, achievement levels, and rewards store
• Help translators get recognized for their contributions through the platform

References

• Original Crowdin discussion announcement: [https://crowdin.com/project/gitlab-ee/discussions/261]
• GitLab contributor platform: https://contributors.gitlab.com/leaderboard
• Account mapping form: https://docs.google.com/forms/d/15XDNFFLhbBJgB4U3u44yRwdiecRAe2ykNl0kRtPFkO0/edit?ts=68000e25
• Contributor platform user guide: https://contributors.gitlab.com/docs/user-guide

How to set up and validate locally

1. Navigate to the translation documentation page
2. Verify that the new "Contributor Recognition and Rewards" section appears after the Crowdin account creation section
3. Check that all links in the new section are working correctly:
   • Leaderboard link: https://contributors.gitlab.com/leaderboard
   • Achievement levels link: https://contributors.gitlab.com/docs/user-guide#contributor-levels
   • Rewards store link: https://rewards.gitlab.com/
   • Account mapping form link
   • User guide link
4. Confirm that the content flows logically from Crowdin setup to contributor recognition information

MR acceptance checklist

This MR:

• Documentation has been updated
• All links have been verified and are working
• Content follows GitLab documentation style guidelines
• No breaking changes introduced
• Addresses the issue requirements completely