Mechanism for shared glossary entries (and other content)

The Stack Glossary consolidates definitions from other repos (notably the Elasticsearch repo), but reusing definitions across other books would introduce additional dependencies that we want to avoid.

This is relatively static content that we want to curate, so we *could* put definitions in the docs repo. Versioning could be addressed by what definitions are included in the versioned docs. Deprecation warnings could still be used, as they specify the version.

But, as @lcawl said, "As we think more and more about how/if content is reused, we might need a bigger solution or even just a versioned shared repo."