Docs: Use civet grammar in snippets highlighting

[Argeento]

You can also use https://github.com/DanielXMoore/Civet/blob/master/lsp/syntaxes/civet.json - https://github.com/shikijs/shiki/blob/main/docs/languages.md#supporting-your-own-languages-with-shiki to use your vscode tooling here

Originally posted by @orta in https://github.com/DanielXMoore/Civet/pull/172#discussion_r1064463706