fix(docs) Fix nested markdown in MDX components

[tesseralis]

Description

Fix formatting of <Pullquote>, <figcaption> and <VisuallyHidden> to handle nested markdown links in them and multi-line content.

Motivation

• Some of our links were broken
• soft-wrapped text is easier to translate

TODO:

• Check the build to make sure things are still rendered correctly

[AishaBlake]

I tried looking at some of these pages locally and I'm not seeing a difference between some of the pages I spot checked. Others, I'm getting this error when I attempt to view a post.

TypeError: Cannot read property 'frontmatter' of null
BlogPostTemplate.render
src/templates/template-blog-post.js:39
  36 | )
  37 | let canonicalLink
  38 | 
> 39 | if (post.frontmatter.canonicalLink) {
  40 |   canonicalLink = (
  41 |     <link rel="canonical" href={post.frontmatter.canonicalLink} />
  42 |   )
View compiled
▶ 16 stack frames were collapsed.
(anonymous function)
/Users/aishablake/gatsby/www/.cache/app.js:67
  64 | const preferDefault = m => (m && m.default) || m
  65 | let Root = preferDefault(require(`./root`))
  66 | domReady(() => {
> 67 |   renderer(<Root />, rootElement, () => {
  68 |     apiRunner(`onInitialClientRender`)
  69 |   })
  70 | })
View compiled

[tesseralis]

The build is failing because it can't parse figcaptions correctly:

<figure>
  <img
    alt="Gatsby swag posts on Twitter."
    src="./images/gatsby-swag-twitter.jpg"
  />
  <figcaption>

Tweets from [Dean](https://twitter.com/DeaNHtiD99/status/1023204484183416832), [Elle](https://twitter.com/ARebelBelle/status/1020044426712735744), [Chase](https://twitter.com/chaseadamsio/status/1021896138503245824), and [Katherine](https://twitter.com/kato_katherine/status/1021967765400211456) showing off their new Gatsby swag.

  </figcaption>
</figure>

For some reason the top <figcaption> tag gets closed when compiling with gatsby-plugin-mdx.

[AishaBlake]

@tesseralis did you want additional feedback on this? Input from the Content team specifically? Something else?

[tesseralis]

@AishaBlake on pause for now since working on other stuff like the theme refactor. Should I close this PR for now?

[AishaBlake]

Sounds good to me! We can revisit later.