Parsec Server migration guide

[mmmarcos]

The Parsec Server installation guide has been recently added to the documentation. We should also add instructions to upgrade a running Parsec Server (specially for on-premise deployments) to a newer version.

Instructions should focus mainly in the steps required to migrate (meta)data to a newer version and the checks to be performed before deploying in production. It should be consistent with the instructions described in the Parsec Server installation.

Some instructions already exists here: https://github.com/Scille/parsec-cloud-saas-deploy/blob/master/README.md