Emit warning when readme contains figures in wrong directory

[jeroen]

There are a lot of pkgdown sites with broken figures on the homepage because most maintainers are not aware that pkgdown only supports images in a few locations since only a few locations also work with CRAN's rendering of readme.md.

This could be documented a bit better and perhaps it would be helpful if build_site() emits a warning when the readme contains figures to relative paths in source folder that are not supported by pkgdown, and a hint to move these to e.g. ./man/figures/

@maelle