Replace `jp` with `jr` where appropriate

[avivace]

Several places in the tutorial still use jp (absolute jump) where jr (relative jump) would be more appropriate and cycle-efficient. jr is properly explained and introduced in Part I - Jumps, so from that point onward it should be safe to replace jp with jr wherever the target label is within the 128 bytes range.

Work has already started on this (#146), but some occurrences remain.

[Les-Wet]

There are some traces of jp in screenshots as well, especially in the Tracing lesson. Would take some extra work, but might be worth updating those as well.