learnsite: Make Exercises usable in frontend only

[juliaogris]

Make Exercises usable in frontend only:

• show one question at a time (with random composition)
• on click Check reveal right or wrong
• no backend at this stage.

Current UX has multiple questions per page, and no Check button

https://learn.evy.dev/foundation/sequence/print/index.html

Consider embedding labs in learn.evy.dev

Lab/Learn: link from landing.

If Lab stays stand-alone, consider:

• Lab: overview (unless we embed with learn.evy.dev)
• Lab: bottom button to next lab (unless we embed with learn.evy.dev)