Add more profiling and benchmarking documentation

[glebm]

On Linux, you can now run:

tools/build_and_run_benchmark.py --gperf devilutionx -- --diablo --spawn --lang en --demo 0 --timedemo

and get this:

[qndel]

https://godbolt.org/z/34h3Woscx This doesn't look good 😛 wrong values + Program terminated with signal: SIGFPE https://godbolt.org/z/jMjx78cqd

Nvm was caused by dividing by 0 (srcWidth = 0), still, the behavior for remainingWidth > 0 differs