Add code font configuration option alongside heading and body in fonts

[adriamontoto]

Currently, the fonts configuration in docs.json supports setting custom fonts for heading and body, but there's no equivalent option for code blocks and inline code snippets. To customize the code font today, you have to resort to custom CSS overrides, which feels inconsistent with the clean declarative approach available for other typography.

It would be awesome to add a code property to the fonts object (with the same family, weight, source, and format fields) so users can configure their monospace/code font directly in docs.json — for example:

{
  "fonts": {
    "heading": { "family": "Inter", "weight": 700 },
    "body": { "family": "Inter", "weight": 400 },
    "code": { "family": "JetBrains Mono", "weight": 400 }
  }
}