Replace `balena.models.supervisor` examples with `device`

[pranasziaukas]

The supervisor model has been integrated into the device class, yet some code examples were mistakenly pointing to the wrong model

Fixes #333

[otaviojacobi]

Thanks @pranasziaukas Would you mind also running python docs_generator.py > DOCUMENTATION.md so that the actual docs file are also updated? I still haven't had the time to make our CI do it automatically for us on merge

[pranasziaukas]

LGTM

[pranasziaukas]

@balena-ci I self-certify!

[pranasziaukas]

@klutchell do you know what's up with the policy-bot: master — Error evaluating policy in balena-io/.github@master: policies/approval.yml?

[otaviojacobi]

LGTM

[pranasziaukas]

@balena-ci retest