docs: add a functionally grouped entry point into the reference

[sgerbino]

From the review result:

The reference guides should break the typelists into layers/sections like: promise_types, buffer_types, stream_types, synchronization_types. Similar for Corosio.

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

The Reference nav entry is a single flat page and the generated index groups only by entity kind (Namespaces, Types, Enums, Functions, Concepts). STYLE_GUIDE rule E3 already states the aspiration. Constraint: this must use existing MrDocs functionality only; the generated index stays as-is. The grouping layer is authored on the Antora side.

• Hand-authored reference overview page in the ROOT module with sections (promise types, buffer types, stream types, synchronization types, executors/contexts) linking into the generated pages via the cpp: macro
• Nav sub-entries under Reference pointing at those sections