Add Guard to Wistia-Doc Dev

One thing that less-than-optimal about development on the doc is that we use somewhat janky workflows to compile coffeescript, sass, and haml, because Jekyll doesn't support them by default but we want to use them for development. This has meant updates to certain files (like layout haml) required a full dev environment restart - not too intuitive.

I'd like to try using the [Guard gem](https://github.com/guard/guard) for dev instead. While it would use the same command for dev, `rake preview`, under the hood it now watches for changes in `coffeescript`, `sass`, and `haml` files. I would love to move all of jekyll build to Guard but all the jekyll-guard plugins are brokie.

Part of this PR is re-organizing the js and stylesheets we should use during dev, moving them to the `_assets` folder. Compiled coffeescript and sass end up in the `javascripts` and `stylesheets` directories, respectively.

One thing I'm not clear on is how this will affect production. The `_site` directory looks the same, so if we were just serving from those compiled static files, all should be good. Just need to double check with @bschwartz :bird: :man: