make it easy to build (with mdbook)

[tshepang]

mdbook expects to find SUMMARY.md inside ./src, so how about renaming src/ to code/
and moving content to src/. I just checked... works well. I see there will ne a need for further small changes. I can submit a PR if this looks like the right idea.

[stevedonovan]

I like this, because then it's easy for everyone to build the book. I was using symlinks - ouch!