• Please check if the PR fulfills these requirements

• The commit message follows our guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

I'll file a separate issue for docs/tests on all features realted to the blog or Markdown/MDX.

• What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)

Feature: frontmatter is now injected as pageProps (behind the scenes, this uses getStaticProps).

• What is the current behavior? (You can also link to an open issue here)

Frontmatter is useless.

• What is the new behavior (if this is a feature change)?

Front-matter is now available in _app.tsx as pageProps, and can be used for all sorts of neat stuff programmatically.

For example:

// src/pages/_app.tsx

import {MDXProvider} from '@mdx-js/react';
import blogComponents from '../features/blog/components'
import eventComponents from '../features/events/components'

function MyApp({Component, pageProps}: AppPropsWithLayout) {
  let components = {};
  // For `.md` & `.mdx` files in the `pages` directory, frontmatter from the file becomes `pageProps`
  if (pageProps.contentType === 'blog') {
    components = blogComponents
  }
  if (pageProps.contentType === 'event') {
    components = eventComponents
  }
  return (
    <MDXProvider components={components}>
      <Component {...pageProps} />
    </MDXProvider>
  );
}

In addition frontmatter values in .mdx pages can be used as input variables for the MDX on that page. For example, the following MDX file:

---
title: The answer to life, the universe, and everything
answer: 42
---

{title} is {answer}

is roughly equivalent to the following:

export const title = 'The answer to life, the universe, and everything'
export const answer = '42'

export const getStaticProps = () => {
  return {
    props: JSON.parse(
      JSON.stringify({ title, answer })
    ),
  }
}

export default function MDXPage({ title, answer }) {
  return (
    <p>
      {title} is {answer}
    </p>
  )
}

• Does this PR introduce a breaking change? (What changes might users need to make in their application due to this PR?)

Nope!

• Other information:
  Closes Feat: Turn front-matter in markdown pages into something useful #132
  Unblocks Build blog list component (Index page) #20, Build blog reader component #21, Build blog filter #22, possibly man others.

Next we will need to validate the frontmatter and do stuff with it.