Add four new resource types for common directory paths and a
convenience setup struct that registers all of them at once.

Add module-level docs for the hook system, enhance doc examples for
macros
with usage syntax, fix typos in cross-references, and hide internal
debugging
macros and mock types from public documentation

Support `&mut T` resource parameters in async chain functions by using
an extract-store pattern that avoids holding mutable borrows across
await
points. Remove the previous compile-time rejection of this combination.

Move the Getting Started section above Writing with Mingling, replace
emoji decorations with plain text, remove unnecessary `use
mingling::prelude::*;` imports from code examples, and update crate
description.

Introduce `StructuralData` sealed trait and `pack_structural!` /
`group_structural!` / `derive(StructuralData)` macros to control
structured rendering separately from grouping. `Groupped` no longer
requires `Serialize`.