Update to Extending doc breaks docs site build

[verythorough]

- Do you want to request a feature or report a bug? docs bug

- What is the current behavior? While preparing to preview a docs edit, I discovered that builds on the netlify-cms-www repo have been failing since https://github.com/netlify/netlify-cms/commit/64b0d4cf068a9d84f8bf74275ffd3f3c84e83ab2. This is because the site is built with Hugo, and it's trying to process the shortcode syntax that's now in the code sample.

- If the current behavior is a bug, please provide the steps to reproduce. Go to https://www.netlifycms.org/docs/extending/ and see that the new version has not published.

- What is the expected behavior? Content on https://www.netlifycms.org/docs should match content in https://github.com/netlify/netlify-cms/tree/master/docs.

I discovered that you can escape the shortcodes by adding /* */ inside them, and it works in the Hugo-built site:

However, the version on GitHub still shows the escape characters:

Ah, the joys of writing markdown for two simultaneous interpreters!

[erquhart]

Great catch @verythorough, reverted in 83a1d34.