Make a `ppx_odoc` that does the comment checking that `ppx_js_style` does

ppx_js_style is the Jane Street style checking ppx. One of the things it checks is that documentation comments have valid syntax using the documentation comment parser from the old version of odoc (octavius).

Now that the parser is no longer a separate library ppx_js_style can't use an up-to-date documentation parser. Now that odoc is getting more widely used there are probably people who want to check their documentation comments without using the other parts of ppx_js_style. So my suggestion is to implement a stand-alone ppx that just checks the syntax of documentation comments.

At the moment I think it would have to live inside the odoc repository, since that is where the parser is available, but it could also be implemented separately if odoc added a parser library. Either approach seems fine to me.