Extend glossary to also be index

To help with discoverability/findability, add key documents for each topic.

Also cross-reference the Concepts document, add basho_bench, and shorten the kv definition.

I want to make the list of references include icons to indicate where the referenced documents live (theory, ops, dev, Basho blog, GitHub) but that can wait for another PR. Barrett is working on the necessary design magic.