Add Previous/Next links to bottom of Effective Dart pages

[kwalrath]

From an internal bug (now closed):
Currently you read a page, it ends, and there is no easy way to go next.
You need to scroll all the way up to move to next page, it is kind of clunky.

We could perhaps use the same scheme as the tutorials, where next/previous is defined by variables in the template.

[Sfshaza]

Fixed.