use sbt-site plugin for doc generation

build.sbt

project/plugins.sbt

src/sphinx/.gitignore

src/sphinx/Makefile

src/sphinx/conf.py

src/sphinx/faq.rst

src/sphinx/gettingstarted.rst

src/sphinx/index.rst

src/sphinx/installation.rst

src/sphinx/intro.rst

src/sphinx/library.rst

src/sphinx/limitations.rst

src/sphinx/neon.rst

src/sphinx/options.rst

src/sphinx/purescala.rst

src/sphinx/r

src/sphinx/references.rst

src/sphinx/repair.rst

src/sphinx/synthesis.rst

src/sphinx/themes/leon/layout.html