Add skeleton of a language reference. (#4808)

The files being checked in here were initially written 2-3 years ago.
They are very clearly incomplete, outdated and, quite often, inaccurate.

The intention of this change is to get an initial skeleton in place for
the large-scale structure of a language reference (for now avoiding
the term "specification" and all the weight it carries).
Future commits should be able to add to flesh out this skeleton and
start to make the document more complete and more accurate.

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