Add a FDB Glossary page to documentation

It's possible I have missed it, but I couldn't find any Glossary/Terminology page in our documentation. It would be super nice to have a place to look for terminology.

No documentation file, test, or entry point is named in the issue; begin by locating the existing documentation structure and terminology used throughout FoundationDB. Done means the documentation contains a discoverable glossary or terminology page covering the project's key terms.

Written by the indexing model from the issue text.