Updated markdown export implementation

- Removed ZIP system for now, until the idea can be fleshed out.
- Added testing to cover.
- Upgraded used library.
- Added custom handling for BookStack callouts.
- Added HTML cleanup to better produce output for things like code
  blocks.

app/Entities/Models/Chapter.php

@@ -5,6 +5,7 @@ use Illuminate\Support\Collection;
 /**
  * Class Chapter
  * @property Collection<Page> $pages
+ * @property mixed description
  */
 class Chapter extends BookChild
 {