Improve the themes appearance on themes.gohugo.io

[leoheitmannruiz]

This PR:

• makes the paths for images absolute, so that they can be displayed at https://themes.gohugo.io/themes/newsroom/. See https://github.com/gohugoio/hugoThemesSiteBuilder#use-absolute-paths-for-images.
• replaces one GitHub emoji declaration with the actual emoji so that it can be displayed at https://themes.gohugo.io/themes/newsroom/.
• adds the demosite key-value pair in theme.toml so that a direct link to the demo site can be generated at https://themes.gohugo.io/themes/newsroom/. See an example https://themes.gohugo.io/themes/hugo-book/.

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