Add a glossary to the docs

As we begin the work on refactoring the docs, it's clear that we are starting to trip over some of the terminology. There are lots of instances when we shift from one term to another to refer to the same thing, which could introduce confusion for the reader.

Here are several of the terms that need to be defined and refined.

• export / export file / export document
• reduce
• merge / combine / aggregate
• assembly model
• assembly / assembly document / assembly AsciiDoc source document / aggregate AsciiDoc / assembled AsciiDoc / assembled document
• converter / convert document function
• exporter / exporter extension