create Audio Skew documentation

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?

Start with the existing Dynamic Rate Control documentation and review how the docs are organized for a possible user-facing addition. Use the requested Robocop audio recordings as the example, and cover audio skew, the 0.05 default threshold, and the quality trade-offs; done means both the user explanation and developer documentation stub are present.