Removing the deprecated `base` repository from more docs

Was discussing the `base` repository with @shykes on IRC and we found a few more references to it within docs.  While I was looking over them, I saw a few more small issues that needed fixing, so I took care of those also (such as backporting a small grammar fix across all the remote api docs and removing a reference on the baseimages page to a contrib/mkimage-centos.sh that doesn't exist).