There are two links in the file "manage_plugins.html" that result in 404 errors when I click on them. The two files, testplugin_v013.zip and Sigil_Plugin_Framework_rev8.epub apparently are no longer available in the repo :
https://github.com/Sigil-Ebook/Sigil/tree/master/docs.
We might want to update those links while we are fixing things.