doc: Update migration docs

[nejraselimovic]

Summary

Updating migration docs to add some clarifications and correct some existing info (related to migrating active sessions, using packet broker, re-programming ABP devices, etc).

Checklist

• [x] Scope: The referenced issue is addressed, there are no unrelated changes.
• [x] Run Locally: Verified that the docs build using make server, posted screenshots, verified external links.
• [x] Style Guidelines: Documentation obeys style guidelines in CONTRIBUTING.
• [ ] Commits: Commit messages follow guidelines in CONTRIBUTING, there are no fixup commits left.

[nejraselimovic]

@laurensslats @neoaggelos a friendly reminder to submit your review so I can merge this week