Document how Git object deduplication works in GitLab

What does this MR do?

This MR adds documentation on how Git object deduplication is implemented in GitLab. Git object deduplication is a new Gitaly feature we are working on.

This is developer-facing documentation to help everybody who is interested understand how Git object deduplication works in GitLab, and to help us (the Gitaly team) judge where we are on the scale from "beta feature" to "GA feature".

What are the relevant issue numbers?

&189 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer