Added a Citation File Format for citation metadata.

• Added a Citation File Format (.cff)-file
  • Contains citation info
  • Is integrated into GitHub (not in GitLab yet)
  • Can be converted into other formats (e.g. bibtex)
  • The list of authors is ordered by number of commits
• Added the actual software citation on the web publications page, see preview
• Added a copy-to-clipboard button for all code blocks on the website for easy copy and pasting (requires JavaScript to be enabled in the browser)

Looks like this on GitHub:

1. Feature description was added to the changelog
2. Any new feature or behavior change was documented?