Add custom Valkey syntax highlighter

[stockholmux]

Is your feature request related to a problem? Please describe.

Right now blog posts and documentation that include simulated valkey-cli code blocks have no highlighting. They're hard to read and understand as they're just grey on black.

Describe the solution you'd like

In working on #305, I discovered Zola allows for custom syntax highlighters via Sublime Text Syntax definition files. We should write our own syntax highlighter and maybe contribute it to Zola.

Describe alternatives you've considered

Leaving code blocks unhighlighted or use something close that would be easier to read than grey on black.

Additional context

n/a