Switch internals to Mdx

[sapegin]

I've made a quick prototype using Mdx as a base for rendering Markdown and examples, and looks like it has some benefits over the current approach:

• The overall architecture seems simpler (this might be because I'm doing it now, and the current approach has five-years-old roots ;-)
• Less code to maintain.
• We'll be able to support both Styleguidst Markdown and Mdx in the future.

Some notes:

• This doesn't mean we'll be able to load Storybook Mdx directly, since we're not going to emulate their components.
• This will be a breaking change even for Styleguidst Markdown, though I expect most files will not require changes.

Related issues: #1497.

Rull request: #1720.

[stale]

😴 This issue has been automatically marked as stale because it has not had recent activity. It will be closed in a week without any further activity. Consider opening a pull request if you still have this issue or want this feature.

[stale]

😴 This issue has been automatically marked as stale because it has not had recent activity. It will be closed in a week without any further activity. Consider opening a pull request if you still have this issue or want this feature.