Variable inheritance works implicitly

Harsh Chourariarequested to merge
hchouraria-docs-ci-variable-inheritance-expansion into master

What does this MR do?

The docs imply that one needs to use dependencies or needs to gain variable inheritance in later jobs, but this isn't true. All jobs in later stages will receive an inherited value without requiring those keywords.

This change clarifies the documentation to reflect the observed behavior. It also adds examples demonstrating how variable inheritance can be turned off explicitly, and how inherited variables may take precedence over other methods of variable definition.

The required format for the .env file is also explicitly stated, based on its its implementation at: https://gitlab.com/gitlab-org/gitlab/blob/2e5b4dbf3ab159f7ea4960d5fe7dfb06c7ed93e1/app/services/ci/parse_dotenv_artifact_service.rb#L55-58

Related issues

Customer ticket: https://gitlab.zendesk.com/agent/tickets/251886

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