Assets consolidation

Hey,

I've done most of the groundwork on this but have a few things to run by you. They are mainly about the `mdoc` process as I'm not that familiar with it.

* I've added a `data-version-tag` attribute to each of the `<body>` tags in the docs and in the `template\header.hbs`. I've added `data-version-tag={{VERSION_TAG}}`, more of a placeholder than anything else, but how does mdoc know what the current version is being created, and can I use it in the header.hbs?
* Kind of similar to above. Currently the `.hbs` files use `{{root_path}}` when setting up the assets paths. I'll need an `{{assets_path}}` which instead will output `../../template/` rather than the path to the docs.
* When creating a new set of docs we'll have to ignore copying over the `assets_` directory. What do you suggest is the best way to do this?

Thanks,
Pat