sync gateway upgrade scenario

Scenario for sync gateway upgrade : Sync-Gateway-upgrade-Testing.doc

^Tasks:

• ^{convert tables into section on https://docs.couchbase.com/sync-gateway/2.1/upgrade.html}
• ^{use content from the above to create a generic section which can linked to from a specific upgrade path}
• ^{pr to update https://github.com/couchbase/docs-sync-gateway/pull/119}
• for Server section, use title Considerations from Couchbase Server Upgrades