docs: add a glossary #407

From the review result:

The Corosio docs have a helpful Glossary, Capy does not.

Source: Boost review result (conditional acceptance), Boost mailing list, September 2026.

No glossary page exists, yet doc/STYLE_GUIDE.md rule C7 requires every term of art to carry a definition or a glossary link. Seed it from the Corosio glossary where terms overlap (executor, affinity, awaitable, strand).