Rework guide about sampling

[Asmatzaile]

I reworked the guide about sampling, which was in 'usage':

• I moved the reference about the existing samples to its own reference page.
• 'Add samples' has also its own guide now; it's in 'audio', next to 'add effects' and 'add synth'.
• 'Lazy loading', related but not exclusive to using custom samples, is another guide in 'audio'.
• In 'usage', there remains a guide about using metatune, called "assign the actual notes to the samples" (suggestions for a better name are welcome)

[trespaul]

If we come up with a better name we can add it later :)