Guide Improvement: Add some musical examples of when double-sharp / double-flat is most useful

[pikurasa]

Current Behavior

Folks, understandably, wonder what's up with double-sharp / double-flat.

We should add some examples to our guide.

Some ideas:

• G# Major as an example: G#, A#, B#, C#, D#, E#, Fx (F double-sharp)
• D# Major as an example, with C and F double-sharp
• Fb Major as an example, with B double-flat
• Bartok's "Whole-tone Scale" Mikrokosmos (Number 136) has double-flats
• Search for other examples from Mikrokosmos: https://en.wikipedia.org/wiki/Mikrokosmos_(Bart%C3%B3k)

The sixth measure of this example has double-flats: