Create a Markdown extension

## Description

A simplified markup language that lets you leverage Ruby functions and partials with a more Markdown-like syntax.

## Motivation

We currently provide a way for users to mix different rendering languages. This is nice and should stay, but it would be even better to have a unified language that lets you leverage ERB with a Markdown-like syntax. This is more user friendly for writers, and allows you to lint content with tools like [stylelint](https://stylelint.io/) without having to write a custom parser.

## Solution

We could extend Markdown to develop a few tags that let users leverage Ruby features without actually writing ERB.

For example:

```erb
# Before

<%= partial 'panel/table.haml', locals: {
data: {
'Group title': [
{
text: 'My title',
link: '/path/to/page/'
}
],
}
} %>
```

```html
// After

```

No files, tests, or entry points are named. Start by locating the existing support for mixing rendering languages in the Ruby static-site generator, then clarify the tag syntax and scope using the proposed examples; done would require an agreed extension that handles the illustrated partial and data case.

Written by the indexing model from the issue text.