Update documentation, adding SLE16 and also products e.g SLES4SAP

[schaefi]

For product specific migrations like SLES4SAP we want to have a handler in terms of a dedicated migration live image such that the needs for the customer can be achieved without influencing the standard OS migration workflow. That we have this opportunity needs to be documented and relates to the PR #333

[schaefi]

In addition the current documentation is specific to SLE12 - SLE15 migrations and uses this wording. As the scope now increases to also SLE15 - SLE16 migration the complete document should be revised or we need to decide how to structure it per SLE version or something