📖 ADR: v1-v3 automatic migration

[Danil-Grigorev]

What this PR does / why we need it:

ADR on automatic migration procedure for provisioning.cattle.io/v1 clusters to management.cattle.io/v3 clusters.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged): Fixes #

Special notes for your reviewer:

Checklist:

• [ ] squashed commits into logical changes
• [x] includes documentation
• [ ] adds unit tests
• [ ] adds or updates e2e tests

[Danil-Grigorev]

@salasberryfin Makes sense, I’ll add a reference and push some changes soon.