R1:cd41b834f71f

Improve Diviner linking

Summary:
Do this somewhat reasonably:

- For links to the same documentation book (the common case), go look up that the thing you're linking to actualy exists. If it doesn't, render a <span> which we can make have a red background and warn about later.
- For links to some other book, just generate a link and hope it hits something. We can improve and augment this later.
- For non-documentation links (links in comments, e.g.) just generate a query link into the Diviner app.