ExportFormatter: Add missing book description and remove extra newlines in markdown export

[czemu]

Hi,

This pull request includes two updates to the app/Entities/Tools/ExportFormatter.php file regarding the markdown export:

1. Added the missing book description in the bookToMarkdown() method.
2. Added a check for an empty chapter description, preventing unwanted extra lines (\n\n) between the chapter name and the page content when the description is empty.

[ssddanbrown]

Thanks for these changes/fixes @czemu!
They are now merged to be part of the next feature release.

I followed this up with 0f9957b to handle the wider range of potential formatting in descriptions via HTML to MD conversion, and to also update testing to cover the changes made.

[czemu]

Thanks @ssddanbrown!