Revise XP Migration guide for minimalism

Following feedback that the XP migration guide is not very useful, the guide was reviewed in a crowdsourced minimalism session. The following issues were identified in that session:

Section 1 is not scannable and much of the information is not clear.

In section 2, the appropriate state for the upgrade is not clear. Some information is missing in the steps.

The Troubleshooting content should be moved to a separate module, and revised to emphasize user focus.