Finish docs/debugging.md

[dwRchyngqxs]

So I started updating docs/debugging.md but I stopped because the PR included other stuff which delayed marking.
I need to explain debugging with RR and spike.

[Fiwo735]

(That probably also involves some changes to .vscode/launch.json and .vscode/tasks.json)

[dwRchyngqxs]

Explainer for skipping all the pk code in spike:
find the symbol (=function) you want and copy the value in the first column of the output of riscv32-unknown-elf-nm -gU <elf> or if you know the name of the symbol, use riscv32-unknown-elf-nm -gU <elf> | grep " <symbol>$" | cut -d" " -f1.
Launch the debugging session with spike --isa=rv32gc -d pk <elf> and type in the command until pc 0 <the_value>.