docs(versions): rework the version info extraction

The docs were relying on the grunt/util module for getting version info
but this was unreliable and full of custom regexes.  This is moved into
a new version-info module that makes much better use of the semver library.
chore(build): refactor build version information
style(jsdoc tags): remove/ammend invalid tags

As highlighted by the new sterner dgeni.
docs(tutorial): display button icons (Previous, Live Demo, ...)

In relation to https://github.com/angular/dgeni-packages/pull/8

Closes #6641