Adds API project cluster documentation

What does this MR do?

Adds project cluster documentation - https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/23922/diffs

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/40473

Does this MR meet the acceptance criteria?

• [-] Changelog entry added, if necessary
• Documentation created/updated
• [-] 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