Add hint about how to use variables

[timqian]

Summary

Failed to figure out in the docs about how to use the frontmatter variables. After trying {{$frontmatter}}, I guessed out it myself. It should save me an hour if the doc tells me how to invoke the variables

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• Bugfix
• Feature
• Code style update
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Other information:

[bencodezen]

@timqian Do you find this example to be more helpful?

[timqian]

@bencodezen Cool, this looks even better

[bencodezen]

@timqian Glad to hear it! I'll go ahead and merge this while updating the example again locally since the suggestion feature is a little buggy with code blocks.