Add some code examples

[JulianGCalderon]

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Adds some code examples to the Concrete book, regarding basic language features.

I've not added documentation on these topics:

• Traits (not in main yet).
• Modules (or standard library).
• Attributes:
  • intrinsic.
  • lang_item.
• Borrowing rules for references.
• Union (low-level, is it worth to document?).

Also, our for implementation is pretty flexible (Go-like), allowing something like this:

for (; x <= 10; x = x + 1) {
    ...
}

I've not documented as I didn't know if we would keep it.

NOTE: Is there a way to preview the book before merging?