/doc/reference/reducers/ (i.e., document reducers beta)

[behoppe]

Task
To document reducers (beta) and then integrate that new content into the appropriate place on the website.

Assignee roles

• Owner: @VoxSciurorum

Additional discussion
This issue replaces the reducer portion of #81, because we're not sure yet where the new reducer documentation will find its forever home (maybe as part of the language specification, or maybe somewhere else). (Please note, @dcurtisatmit.)

[VoxSciurorum]

I have the wrong pathname in the pull request, /hyperdoc instead of /reducers. I'll try to rename the file when I edit it in response to review comments.