This cleans up most of the warnings from sphinx (broken :doc: links,
broxygen role misuses, etc.). The remaining ones should be harmless,
but not quick to silence.
I found that the README for each component was a copy from the actual
repo, so I turned those in to symlinks so they don't get out of date.
I edited the text little bit, reorganized the structure somewhat and
extended some parts. I've also simplified the tests a bit, using some
of the BTest tweaks commited in parallel.
