home page fix (#551)

* explicit site.url in config

* infra for custom css

* moves home item to html layout

* adds baseurl to site config

docs/_layouts/default.html

@@ -25,6 +25,7 @@
       <h1 class="project-name">{{ site.title | default: site.github.repository_name }}</h1>
       <h2 class="project-tagline">{{ site.description | default: site.github.project_tagline }}</h2>
       {% if site.nav %}
+        <a href="{{ site.baseurl }}/" class="btn">Home</a>
         {% for item in site.nav %}
           <a href="{{ item.url }}" class="btn">{{ item.title }}</a>
         {% endfor %}