Expose compileMarkdown as public API

[samselikoff]

• closes Make the markdown renderer w/syntax highlighting a public function #318

[samselikoff]

Use case here was rendering Markdown content in Mirage's blog (example).

AddonDocs sites should be able to do this using the same Markdown renderer used by AddonDocs' own components.

The implementation here moves the compile-markdown function into a new addon-docs-shared subdirectory, which is acting like its own npm package. This is so that it can be require()'d by node scripts, and also Auto Import'd by Ember app code. This is to support run-time use cases (like Mirage's blog).

Test included in the dummy addon.

For more history see this Auto Import issue.