Adding dynamic attribute constraint guide

Categorizing classes and adding JSDuck "guides" as spaces to centralize documentation on specific topics.  To create a guide, you can follow the jsduck documentation here: https://github.com/senchalabs/jsduck/wiki/Guides

To use the guides in documentation, you can link them to the sidebar with the aside tag as `@aside guide [guidename]` or directly in comments with `[linked text](#!/guide/[guidename])`.

Note: In the future we may want to have the builddocs script automate the creation of the `categories.json` and `guides.json` as they'll become cumbersome and take effort to maintain by hand before too long.