Exclude node_modules from Jekyll build

[andylizi]

Before this change:

$ bundle exec jekyll build
Configuration file: ./webassembly-website/_config.yml
            Source: ./webassembly-website
       Destination: docs
 Incremental build: disabled. Enable with --incremental
      Generating... 
                    done in 8.388 seconds.
 Auto-regeneration: disabled. Use --watch to enable.

After this change:

$ bundle exec jekyll build
Configuration file: ./webassembly-website/_config.yml
            Source: ./webassembly-website
       Destination: docs
 Incremental build: disabled. Enable with --incremental
      Generating... 
                    done in 3.13 seconds.
 Auto-regeneration: disabled. Use --watch to enable

Incremental watch mode also sped up tremendously.