Feature request: Ability to have footnotes appear in margin in paged layouts

Currently, footnotes appear in a given location based on the document type.

```
all footnotes are designed to appear:

paged: in an area of the bottom page margin
slides: at the end of the page
plain: at the side, next to their first reference
```

However, it would be great to have the option to allow footnotes to appear in the margins for paged layouts as well. As with margin content, it would be great to have the option to set footnotes at the right or in the outer margins (most common for print layouts).

The issue names no files, tests, or entry points. Begin by locating the paged-layout footnote handling and existing margin-content behavior, then determine how right and outer-margin placement should be selected; done means paged footnotes can use those placements without changing slides or plain layouts.

Written by the indexing model from the issue text.