Add a 'Migration' section to the Magma documentation page

On the following page, we should add a section called 'Migrating a Couchstore bucket to Magma' at the end:

https://docs-staging.couchbase.com/server/7.1/learn/buckets-memory-and-storage/storage-engines.html

Following is the content:

Migrating a Couchstore bucket to Magma

If you have an existing Couchstore bucket that you would like to migrate to Magma, you have to basically create a new bucket with Magma storage engine and then copy the data.

To copy the data you can either use XDCR or you can backup the Couchstore bucket and restore to the new Magma bucket.