Release docs to a gh-pages branch

A proposal of what it could look like. Thoughts on approach welcome.

As part of the `build:and_release_if_updated` task also build a static version of the templates with a couple of minor variants. This will let us show people what the template looks like in its barest form. It also copies in a copy of all the pkg tar files so that people can download a version of the compiled template.

An example of what the baseline gh-pages branch can be seen here: https://github.com/alphagov/govuk_template/tree/gh-pages-idea
Incorporated in eae87f3