authoring guide

Reference docs surfaced through guides { path: "core/authoring" } and man authoring. The content lives in guide.md; this README is just the asset summary.

The four rules that make authored content usable by somebody else: a callable is a typed function, it says what it is in --! directives, a value is a Field.<kind> carrying its description, and a metatable-dispatched surface declares its members with --!members.