translationproject: Improve the description block

- Rename "Description" to Instructions
- Unfold everything by default
- Rework the layout to make more sense
translation: Rename 'Description' to 'Instructions'
Cleanup: Use proper syntax for paths
Document changes in custom CSS handling

Since e95286d90c8d0077ccc1117021c3856a628c0ef2 the custom.css file is
bundled with the main file, URLs are now relative to the parent css
directory.