docs: Big Picture link is broken

[d-koppenhagen]

The "Big Picture" Link here is broken.
It links to:
https://github.com/db-ui/elements/blob/main/doc/architecture.md

[mfranzke]

Sadly this document hasn't ever been filled with content, we've only documented a lot of aspects in our internal wiki. That for the easiest for the moment is to remove it and follow up with an ADR for new decisions; we'll even give an overview regarding the big picture after finishing the evaluation of some of the next decisions.