Fix nix syntax so that snippets included out of context render properly

Example:

```nix
mkDerivation = args: super.mkDerivation (args // {
  doCheck = false;
  enableLibraryProfiling = false;
});
```

Used to highlight the `=` as illegal syntax, which would be true if it
were the only thing in a nix file. So let's just ignore that stuff and
turn it into normal text.

config.toml

@@ -11,9 +11,8 @@ title = "jade's website"
 description = "computers i guess"
 
 [markdown]
-# Whether to do syntax highlighting
-# Theme can be customised by setting the `highlight_theme` variable to a theme supported by Zola
 highlight_code = true
+extra_syntaxes = ["syntax"]
 
 [extra]
 # Put all your custom variables here