create Audio Skew documentation

[markwkidd]

This might be added to an existing doc, or could be its own.

User docs:

• Explain that audio skew is one of the the libretro technologies that allows emulator cores to recreate gameplay across very different hardware
• Specifically audio skew....
• Use Robocop as an example -- with audio recordings -- and explain the emulation quality compromises that come with and without the audio skew
• RetroArch's design philosophy is to encourage smooth entry into the emulation and therefore RA uses a relatively high default audio skew threshold of 0.05, etc.

Developer docs:

• Perhaps stub out an addition to the Dynamic Rate Control doc and expand the scope to also include audio skew?

[markwkidd]

Per radius (edited):

• With the default RetoArch audio skew, a person who has never played RoboCop using a modern display will get incorrect speed and incorrect music pitch, but also smooth scrolling with no judder.
• With an Audio Skew of 0.033333334 the user will get correct speed and correct pitch but will experience display judder.