Add AST section to examples

[pedrobslisboa]

41 files where: 20 are pngs and 8 are old examples files deleted. So don't be scared

Description

This is the first part of adding the ppx-by-example to ppxlib
I will divide it into PR so we can review it better, focusing on the explanation and typos without having tons of files to review. It is really necessary that this section is easy to understand and is aligned with ppxlib.
@NathanReb Can you create a ppx-by-example base branch? So I can point this one to it, and after all merges (Other sections) on this branch, we move with the entire ppx-by-example branch to main?

AST

This PR adds the AST sections AST / Building ASTs / Destructing ASTs

How to test it (It's important to check it not only by code but also as styles)

• At this branch run

   make doc
   open _build/default/_doc/_html/ppxlib/index.html

• Access Examples
• Access each section:
  • AST
  • Building an AST
  • Destructing an AST