Add the ability to provide custom layout

[vjeux]

When I designed the system, I added the ability to provide a custom layout that would use arbitrary JavaScript to render some custom pages. This is why files are called "DocsLayout.js", "BlogPostLayout.js"... This ability to customize it was ripped out during the migration to Docusaurus but I need it for the project I'm working on right now (that should remain unnamed!).

This adds back the ability to do it in a way that fits the third party system. In order to provide a new layout:

1. Add a layout field in the header of your markdown file:

---
layout: mylayout
---

2. In your siteConfig, add

  layouts: {
    mylayout: function({React, Marked}) {
      return class extends React.Component {
        render() {
           return React.createElement('div', {}, this.props.metadata.layout);
         }
      }
    }
  }

I think that it's a reasonable to add and would unblock me :)

[JoelMarcey]

Nice. I like that we can just plug in a layout by using configuration. Thanks!

I added a task to add this to our docs.

And I know what the unnamed project is :P :-)

[JoelMarcey]

I also like that this is not a breaking change as well since it is, for lack of a better phrase, opt-in.