Move `images/`, `js/`, and `css/` to `assets/` path

[briandominick]

The conventional way to package a Jekyll theme (and maybe the required way to do it as a Ruby gem?) is as follows:

├── _layouts/
├── _includes/
├── assets/
│   ├── css/
│   ├── images/
│   └── js/
├── _config.yml
├── asciidocsy.gempsec
├── Gemfile
└── README.md

You won't catch me using Markdown for a Readme in this lifetime, but the rest seems worth sticking to. This is basically a deprecation notice on the lack of a base directory assets/. I did this originally because LiquiDoc Ops had a concept of assets as well, and I did not want them to get confused. (Additionally, on some level it bothers me to call image, JS, and CSS files "assets" but exclude template files -- this could just be me.) I believe the concept of content assets is coming out of LDOps anyway, so it just makes good sense to standardize around this method and call the site files "assets".