Implement documentation generation for components

Adds the `preview` element from docgen generated documentation to the `overview` variable for mustache templates. One open question/comment is the `generate_site.dart` should expect a working directory of `Pixelate` not `Pixelate/bin`. 

To use the `components.json` needs to disable `flex_panel` since it does not follow the components folder structure.

```
  {
    "name": "Layout",
    "components": [
//      "flex_panel",
      "grid_panel"
    ]
  },

```