Several fixes and improvements

Thanks Hadley for this great package!
I encountered a few issues when applying staticdocs to my package.
I propose the following fixes and improvements that should benefit to staticdocs users:
- fix empty sections
- fix Authors on index page
- fix inside-r links
- fix Value section formatting
- keep only high level plots

Details are in the commit messages. The 8 commits adress distinct issues an can be applied separately.
See the result applied to my package here: <https://alea.bordeaux.inria.fr/biips/rbiips_doc>