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.
This, I'm not so sure.
`[Icon]({attach}icons/icon.png)`
and
`[Downloadable File]({attach}/downloads/archive.zip)`
behaving differently sounds confusing. I think they both should preserve the folder structure. IOW, the latter should output `./downloads/archive.zip` rather than `./archive.zip`.