Update site documentation and remove docs from source for public header files.

I've updated the docs for emscripten.h, html5.h, preamble.js, and removed the in-source comments from those files. I've also added the prebuilt HTML for those files.

In the headers I've added comment like:
 * Documentation for the public APIs defined in this file must be updated in 
 *    /emscripten/site/source/docs/api_reference/emscripten.h.rst
 * A prebuilt version of the documentation is available at: 
 *    /emscripten/site/build/html/docs/api_reference/emscripten.h.html 

The changed header files have not been tested. But I've created separate commits so you can choose to keep or not as you will.