feat(docgen): typeset links as footnotes in PDF

When reading a PDF printed on paper or on a reMarkable tablet, it's not possible to see that there even is a link in a PDF. Make this more visible by typesetting the link URL as a footnote.

This is not done on HTML output as web pages are read on browsers that make links easy to see.

This is the first time the AST is transformed by docgen differently based on the output format. The decision of what should be done is a policy decision, best done at the topmost level: in the main function of docgen.

The result of that decidion (turn links into footnotes or not) is communicated from docgen main into the ast.rs module via a new Style struct. This mechanism can later be extended for other typesetting style decisions (e.g., what fonts to use).

Closes: #70 (closed)