Fix order of REST API global nav entries

Add navigation item to global navigation

• Link to the merge request that introduced the new page:
• Give the MR a descriptive title.
• Optional. Link to the review app showing the new page: https://docs.gitlab.com/ee/api/usage_data.html

Issue TW OKR: Check content of REST API resources docs (gitlab#499800)

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In the global nav the "Service Data" entry is not in alphabetical order. Also the title "Service Data" does not match the page's title "Service Ping API".

Proposal:

• Move entry to the correct location.
• Rename the entry "Service Ping" (to match the page's title)