Consider building docs with a static step, instead of with every app server launch.

[kfogel]

Right now, etl/leie/leie/serve.py runs pandoc to convert api.mdwn to api.html at application launch time. This means server launch has a dependency on pandoc, and it means that the usual make-type check to see whether api.html is younger than api.mdwn doesn't happen, because we haven't bothered to rewrite make in python.

Should we build docs in a static build step instead?