Make dd-globals extendable via app config

[itpropro]

Currently, the @nuxt-themes/config repo is deprecated and we should leverage the native Nuxt 3 app.config feature. This is fine for SFC usage with the useAppConfig() composable for global variables, but for extending the document-driven globals, we still need to add a _theme.yml (or another name) and reference it in the content configuration. To avoid having so many config files and splitting up global variables, it would be good to be able to use the app.config.ts and configure theme settings (especially the layout override) like this:

export default defineAppConfig({
  theme: {
    layout: 'article'
  }
});

[github-actions[bot]]

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