mdast documentation

Spin-off from GH-15.

Some points I walked into:
- I’m using `function foo(...)` and the like for user-defined functions, e.g., `function attacher(mdast[, options])`, these are still visible in was (e.g., for autocompletion), that seems a bit superfluous;
- `wat mdast process` doesn’t show the list of parameters, it ends at `Parameters` and doesn’t show the rest;
- E-mail links are mangled to HTML entities, e.g., `Written by Titus Wormer\ntit...`;
- It seems I cannot enter `wat mdast fileSet/`, for some reason?
- Why does `wat mdast mdastrc` show up? Weird!

P.S. Really cool that it’s possible to search in documents, e.g., `what mdast man mdastnode7 delete` works great :smile:.