Add documentation for -ppx

One thing that I'm not sure of is whether to include documentation for Ast_helper and most of its siblings in the pdf (also text &c) manual. Ast_helper alone takes twenty pages. This is not a problem for HTML manual.

I frequently look up signatures in the HTML manual, so it's useful even if it's not documented. However I'm not sure if that is the case with linear manuals.
Then maybe we could start slow and recommend that the interested people go read the `.mli` file directly. We could here just briefly describe the various files and their purpose, without including any generated HTML documentation.