DOC ONLY: clean up plugin documentation

The old "sample_plugin" page was stuffed with superceded autotools
build information, so it morphed into an "add a new plugin" page based
on the emacs-lisp plugin generator.

Before sending hate mail about emacs, please *look* at the new
document: you'll find running the plugin generator hard to tell from
running a shell script.

Change-Id: I84da45675e838c05faeca05c8f7be45d8c7bff13
Signed-off-by: Dave Barach <dave@barachs.net>