FR: Add sample config entry migration

[bj00rn]

Checklist

• [X] I have filled out the template to the best of my ability.
• [X] This only contains 1 feature request (if you have multiple feature requests, open one feature request for each feature request).
• [X] This issue is not a duplicate feature request of previous feature requests.

Is your feature request related to a problem? Please describe.

A sample config entry migration would be nice to have.

Describe the solution you'd like

Add sample config entry migration https://developers.home-assistant.io/docs/config_entries_config_flow_handler/#config-entry-migration

Describe alternatives you've considered

n/a

Additional context

n/a

[ludeeus]

https://github.com/home-assistant/example-custom-config/tree/master/custom_components would be a better place to add it