Move migration topics out of Server docs and into Tutorials

The Migrating section under INSTALL & UPGRADE contains two topics that don't really belong there. Specifically, these two topics relate to migrating to Couchbase from other databases. This type of information is best suited for the Tutorials site, where it can be more rapidly iterated upon, taking into account things like advancements/complications in each new release of Couchbase, as well as new releases of the migrating database.

• Migrating from Apache CouchDB
• Migrating from Relational Databases

Note: There is a third topic in the Migrating section, but it has to do with migrating bucket data. This topic should be moved somewhere else; likely to the XDCR sections. This work will be tracked in a different ticket.