Clean up passive voice in mender-docs and enable CI checks

After the mender-docs migration, a leftover task is cleaning up all the passive voice in our documentation.

This will enable us to add the passive-voice checker to CI, and then enforce this on every PR from now on, keeping the documentation up to standards.

This epic will split this task up into pieces, whereas step-1 is enabling the passive-voice checker with a whitelist on the current sections which have been cleaned up (Get-started, Overview). And then piecewise clean up every following section and add the CI checker to this section.