Add WIT example section

[catamorphism]

In comments on #287 , @vados-cosmonic suggested I open an issue to discuss the change.

I think the existing "WIT Overview" section is a bit overwhelming to beginners, especially those who are not familiar with IDLs. In the PR I proposed adding a separate example section, before the overview section are. Some questions are:

• Should the example and overview sections be separate?
  • One approach is to keep it as it is and try to make the overview section more readable.
  • Another is to replace the overview section with an examples section and link to other material (if it exists) to serve as a WIT reference.
  • Personally I favor separate sections that cover the material differently.
• What kinds of examples are helpful? In the PR I included stripped-down versions of the clocks and filesystems interfaces, but @itowlson commented that this is still pretty complex, which is fair enough. What's something that shows a few more features than the calculator example, but isn't overwhelming?

[catamorphism]

Done in #287