Images in RFCs: the one in-repository image is broken and needs fixing, and external images should be moved into the repository

1. #2856 has an image which is kept in the repository (good!), in the resources directory adjacent to the text directory. But the image doesn’t make it through the RFC book build process, and so https://rust-lang.github.io/rfcs/2856-project-groups.html contains a broken image reference.

2. #1398, #1644, #1824 and #1866 have images hosted on diverse third-party sites. This is bad, because those images can break at any time by deliberate action or by accident. The ones in #1644 and #1824 are even worse because they’re http: URLs, and some popular browser configurations block mixed display content, so they’re already broken.

While we’re at it, #1866 has bad alt text and #1398 has bad markup (<img> is a void tag, there’s no such thing as </img>, and the content between the two should be reworded and put as alt text—yeah, no link, sorry—and some of that content should also have been removed at publication time).

I aver that all images should be in this repository. Something evidently needs to be done about the build process so that this can work. We want images to work both in GitHub’s UI and in the published book. This suggests to me that the resources directory should probably be moved to a child of the text directory (though I know not whether that will be a sufficient action), because the alternative will entail URL rewriting of ../resources which is probably more effort than it’s worth.

I suggest also that image filenames should be branded with the RFC number on publication; in cases like #1824, a subdirectory to group its 18 images is called for.

Review the RFC book build process and the resources and text directories, then inspect the image references in RFCs #1398, #1644, #1824, #1866, and #2856. Done means repository-hosted images render correctly in both GitHub's UI and the published book, with corrected alt text and markup where noted.