feat(docs): add VM migration spec

[MathieuRA]

Description

Current behaviour

• Migrating a VM that has VDIs on a shared SR from host A to host B must trigger a "Simple VM Migration" trigger a "Migration with Storage Motion". (wrong behaviour only for migration from the home view)
• Migrating a VM from host A to host B with a destination SR must trigger a "VM Migration with Storage Motion" and move VDIs to the destination SR, regardless of where the VDIs were stored do not move VDIs if they where on a shared SR before the migration.
• A destination SR is required (even if VDIs are on shared SR) in case we want to choose a network for the migration.

PR that fix behaviour: #7360

Checklist

• Commit
  • Title follows commit conventions
  • Reference the relevant issue (Fixes #007, See xoa-support#42, See https://...)
  • If bug fix, add Introduced by
• Changelog
  • If visible by XOA users, add changelog entry
  • Update "Packages to release" in CHANGELOG.unreleased.md
• PR
  • If UI changes, add screenshots
  • If not finished or not tested, open as Draft