Add demosite for "Demo" button on themes.gohugo.io (#891)

### Description

At https://themes.gohugo.io/themes/hugo-coder/ there is no direct link
to the demo site. This PR adds the demosite variable (idk what its
called) in theme.toml, which is necessary for the "Demo" button. See
https://github.com/gohugoio/hugoThemesSiteBuilder#theme-configuration.

Sorry for the issue I accidentally opened and the PR I accidentally
closed hehe

CONTRIBUTORS.md

@@ -139,4 +139,4 @@
 - [Jens Rantil](https://github.com/JensRantil)
 - [Muqeet Malik](https://github.com/mmalik23)
 - [Sammy44nts](https://github.com/sammy44nts)
-  
+- [Leo Heimann Ruiz](https://leo.heitmannruiz.org/)

theme.toml

@@ -3,6 +3,7 @@ license = "MIT"
 licenselink = "https://github.com/luizdepra/hugo-coder/blob/master/LICENSE.md"
 description = "A simple and clean blog theme for Hugo"
 homepage = "https://github.com/luizdepra/hugo-coder/"
+demosite = "https://hugo-coder.netlify.app/"
 tags = [
   "blog",
   "minimal",