[Parisienne] Pull in block starter content from dedicated directory

[jffng]

Following #16, this PR separates starter content in the Parisienne theme from block template parts — which should conceptually be reserved for aspects of the site that are used across pages, not demo content or page templates.

To test:

1. Create a fresh WordPress site
2. Check out this branch and clone it into your site's themes directory
3. Install the Gutenberg plugin
4. Enable the "Full Site Editing" experiment in the Gutenberg > Experiments panel.
5. Activate Parisienne from within Appearance > Themes.
6. Go to Customizer and hit Publish
7. Visit the site and ensure that the Home, About, and Contact pages have loaded with content in each of them.

[kjellr]

This is cool! Like #16, I think this seems like a decent approach forward, but I'm not totally sure if file_get_contents() is the right implementation. So let's hold off for something else's eyes before we merge. Left one other minor question inline. 👍

[jffng]

This was a nice thought, but content like this should really exist in a pattern if at all in the theme.