rewrite the documentation index page

Convert from HTML to Markdown to simplify extending the page.
Mention the docs in mruby's source code and list (future) articles.

Author: cremno

docs/index.html

@@ -0,0 +1,23 @@
+---
+layout: default
+title: mruby - docs
+---
+
+# Documentation
+
+At the moment the source code ([`mruby/doc/`](https://github.com/mruby/mruby/tree/master/doc)) includes further documentation about compiling, configurating (`mrbconf`), debugging (`mrdb`), and extending (`mrbgems`) mruby.
+
+## Articles
+
+<div>
+{% for page in site.pages %}
+  {% if page.categories contains 'articles' %}
+    {% capture article %} - [{{ page.title }}]({{ page.url }}) {% endcapture %}
+    {{ article  | markdownify }}
+  {% endif %}
+{% endfor %}
+</div>
+
+## Components
+
+- [Core](Core.html)