config: migrate markdown highlighting to Zola 0.22 schema

The nixpkgs bump to nixos-26.05 pulls in Zola 0.22, which replaced the
syntect highlighting backend with Giallo and changed the config schema.
Replace the removed `highlight_code` key with a `[markdown.highlighting]`
table using the `one-dark-pro` theme.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

config.toml

@@ -23,7 +23,8 @@ include_path = false
 include_content = true
 
 [markdown]
-highlight_code = true
+[markdown.highlighting]
+theme = "one-dark-pro"
 
 [extra]
 list_pages = true