Add spotlight matRad

matRad is an open source radiotherapy research and education software developed & maintained at DKFZ.

• 👉 Review link

Closes #367 (closed)

Checklist for authors

• Add a link pointing to the review app

Checklist for reviewers

• Reference the corresponding issue in MR
• File size of images is not too large
• Update date_added attribute
• Centres are referenced by official names
• License is named by SPDX identifier if possible
• All links are pointing to the right place
• Code blocks and headlines surrounded by newlines

/cc @frust45 @konrad @jandt-desy @christian.meessen