Fix variables precedence: dotenv variables overwrite all other variables

What does this MR do and why?

Fix the doc of https://docs.gitlab.com/ee/ci/variables/index.html#cicd-variable-precedence

Currently the doc puts dotenv variables in lower place than they actually are in the precedence order.

Actually, variables from dotenv report have the highest precedence and they overwrite any variable with the same name that was set. The overwrite even variables set while triggering the pipeline, schedule pipeline variables, project variables, etc.