add syntax highlighting for Varnish code blocks
minor #4481 [Cookbook][Cache] add syntax highlighting for Varnish code blocks (xabbuh)

This PR was merged into the 2.3 branch.

Discussion
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[Cookbook][Cache] add syntax highlighting for Varnish code blocks

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets |

Commits
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9a52d00 add syntax highlighting for Varnish code blocks