Tag for examples/demo pages

Right now, it's pretty difficult to deal with demo pages. Relative paths and versioning don't work well (see #116). There is no way to add an example as a docObject, and it's very difficult to use examples when they are part of a dependent project (like for DocumentCSS.com). For live style guides, it was requested that examples could be done inline: #118.

To bring demos up to the standard of other tags, this is the minimum feature set:
- [ ] `@example` tag (or something similar) that generates something similar to a `@page`
- [ ] Template specifically for `@example` tags
- [ ] `@iframe` and `@demo` tags can accept a docObject or a path

Nice-to-haves:
- [ ] Configuration of static resource dependencies
- [ ] Rich inline processing (see #118)