Bug 35272: Add padding above vendor contracts section

Bug 35272: Add padding above vendor contracts section

This patch makes minor updates to the CSS and markup of the vendor
detail page so that the different page sections have the correct
margins.

To test, apply the patch and rebuild the staff interface CSS.

- In Acquisitions, locate a vendor and add at least one contract if
  necessary.
- View the vendor details page. Each section (Vendor details, Ordering
  information, Contracts) should have comfortable and consistent
  margins.

Signed-off-by: Caroline Cyr La Rose <caroline.cyr-la-rose@inlibro.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
(cherry picked from commit ce9cd1458c4417e6e52e87df5fe8baeeb6c2d7dd)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>