Documentation example component

[atepoorthuis]

Right now we are using a somewhat convoluted strategy to include both the code for an example as well as its actually rendered state.

So this:

The `vgg-rectangle` mark is used to plot rectangular elements.

<div style="display: flex;
	justify-content: space-around;
	align-items: center"
>

<div>

<MarkRectangleSimple />

</div>

<div style='width: 40%; height: 100%;'>

```html
<vgg-rectangle
  :x="row.year"
  :y1="row.population"
  :y2="0"
  :w="2.5"
  fill="#c66366"
/>

``` Turns into: 

But it doesn't help with the readability of the .md and also requires us to create another adhoc component with the same code in docs/.vuepress/components/. Perhaps this would be a good case for creating a component that takes a valid vue-gg graph as one of its props and displays both the highlighted syntax as well as the rendered svg?