Redo and add more documentation to gh-pages.

Install wintersmith globally and run `wintersmith preview` or build web and open build/gh-pages.

You'll probably notice this links to release zip files. I'd like to use github releases which for now will be a manual process:

1. Update moz central
1. `git checkout <SHA IN MOZ CENTRAL>`
1. Run `node make dist` to generate the zip and update pdfjs.confg
1. Create a PR with pdfjs.config changes
1. Create github release
   2. add release notes
   2. attach zip generated from above
   2. mark as pre-release
1. Remove pre-release flag from previous beta release
1. Eat cake