sgi page linked to by documentation is down

There are links in the doc that now redirect to some unrelated site. For example [here](https://www.boost.org/doc/libs/1_73_0/libs/iterator/doc/generator_iterator.htm) the links to Input Iterator and Generator. The [boost copy](https://www.boost.org/sgi/stl/Generator.html) of those docs should probably be used.