[Documentation] Expand upon design docs

[beefchimi]

We can do more to explain why Earwurm is built the way it is.

For example, we should elaborate on the scratchBuffer substitution for failed fetches. Something like:

If we failed to load the sound, return a “scratch buffer” so that we can still register an ended listener and call .stop() on the source. Since Stack will emit an error event with the Sound > id, consumer’s can listen for that event and perform their own handling.