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.
no need for this line, the entire repo has a license