Replace make ext target and its documentation
With the ext/ subdirectories being an integral part of the repo, as they were added with git subtree, there is no need to init/deinit/clean etc. When this repo is checked out, the test and build dependencies are also checked out. make ext reads the file ext/dependencies.lisp-expr and performs a git subtree pull for all packages in ext to update them to the specified treeish (use 'master' to update to the latest HEAD)