Let documents {attach} static files. Fixes #1019.

Until now, making static files end up in the same output directory as an article that links to them has been difficult, especially when the article's output path is generated based on metadata. This changeset introduces the `{attach}` link syntax, which works like the `{filename}` syntax, but also overrides the static file's output path with the directory of the linking document.

It also clarifies and expands the documentation on linking to internal content.
No worries. I wasn't at all bothered by your question. Just explaining my reasoning. In any case, I don't feel strongly about this one, because either approach would be readable and reasonably performant.