Add 'Cards' docs

[janfaracik]

Adds some guidance for using cards in Jenkins.

Submitter checklist

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Link to relevant issues in GitHub or Jira
• Link to relevant pull requests, esp. upstream and downstream changes
• Ensure you have provided tests - that demonstrates feature works or fixes the issue

[timja]

LGTM (not ran locally)

[uhafner]

Thanks for starting this topic! For me as a plugin author a lot of things are missing, maybe it makes sense to add "in progress" message for this topic.

• Where should I use cards? (On which pages?)
• How can I create cards?
• How do I set the size (or shouldn't they use a predefined grid?)
• Does the expand work out of the box or do I need to program that on my own?
• Do the cards automatically adapt to the screen size? (So will have something in the style of https://muuri.dev/?)