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.

[prmerger-automator]

@JFolberth : Thanks for your contribution! The author(s) have been notified to review your proposed change.

[Court72]

@juliakm

Can you review the proposed changes?

Important: When the changes are ready for publication, adding a #sign-off comment is the best way to signal that the PR is ready for the review team to merge.

#label:"aq-pr-triaged"
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