Update variables.md to elaborate on variable syntax usage.

[JFolberth]

This pull request includes updates to the documentation in docs/pipelines/process/variables.md to improve clarity on variable usage in Azure DevOps pipelines. The most important changes focus on refining the guidance for using macro and template syntax.

Documentation updates:

• Clarified that macro syntax should be used for providing secure strings or predefined variable inputs for a task.
• Expanded the explanation on template variables, emphasizing their use for fully injecting variable values at pipeline compilation and noting that template syntax should not be used for sensitive values.