doc: Refactor coverage setup validation CY-6365

[prcr]

Refactors the coverage documentation to group together all setup validation steps, ordered to make it easier for users to catch any setup issues as early as possible during the validation.

Together with https://github.com/codacy/codacy-coverage-reporter/pull/406, these improvements will give way to more detailed troubleshooting instructions for the most common coverage setup failure scenarios (see CY-6263).