Implement docs build target

[morganava]

We need to build:

• [x] Rust docs with cargo
• [x] doxygen docs for the generated libgosling.h C header
• [x] website html

and eventually:

• [ ] javadoc for JNI interface
• [ ] w/e Python uses for Python interface
• [x] doxygen docs for C++ hpp header

[morganava]

initial docs target implemented in 6ff86c6ad27950d90c70e267f6bd1e28b8e033c5

[morganava]

we also want to build a simple website for specs, news, links to source and docs, releases, etc.

experimenting with a simple mustache-basesd template generator.

[morganava]

website building and deployed in ddede0abc20869ff410a2020c8bf8ce67df7e5b5