Allow assembly-slug document attribute to control filename used for PDF

Currently, the filename for the root-level 1 PDF is generated based on the navtitle. We call this value the "slug". It should be possible to override this value using the assembly-slug document attribute on the page referenced by the level 1 entry in the nav. This behavior gives the author a bit more control over the filename in cases when it ends up being awkward.

The filename for the root-level 0 PDF will remain as index.pdf.

As part of this issue, we may want to revisit how the default slug is computed. All spaces and dots should be converted to hyphens and all other non-alphanumeric characters should be dropped. Any repeating hyphens should then be consolidated. We want to keep it as simple as possible to make it easy to reason about.