Audio chapter: introduce "pitch"

There has been a discussion about introducing "pitch" in the Audio chapter.

@nitro2k01 :

> Actually, rather than omitting pitch from the document, maybe using pitch as the preferred term instead of frequency is useful. Consider especially the noise channel, which for some settings doesn't have a clearly defined frequency, in terms of what the human ear can perceive. (For example 15-bit polynomial setting will produce a cycle that can be more than a second long.) Yet, it would be correct to say that we can vary the pitch of the sound.

This issue is to give us the chance to continue that conversation: https://github.com/gbdev/pandocs/pull/350#discussion_r979423928