Build a canonical Concepts & Terminology reference

Terms like service, route, addon, sandbox, cluster, instance, default route appear across many pages with subtly different framings. The /terminology page exists but isn't exhaustive.

Make Terminology the canonical definition, and on every other page, link the first mention of each term to it. This is a one-time effort and transformative for agents doing cross-page reasoning.

Impact: High. Directly improves the accuracy of agent-generated answers.