Add Git object deduplication development page to global navigation

What does this MR do and why?

Add How Git object deduplication works in GitLab page to global nav.

Screenshots, screen recordings, or links to review app

• Before: https://docs.gitlab.com/ee/development/git_object_deduplication.html
• After: http://eread-add-git-object-deduplication-nav-item.35.193.151.162.nip.io/ee/development/git_object_deduplication.html

Merge request acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this merge request.