add work-in-progress unstable book chapters

These are very bare-bones, only intended to provide some documentation
of what these feature gates are and aren't yet implementing.

Author: dianne

src/doc/unstable-book/src/language-features/ref-pat-eat-one-layer-2024-structural.md

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+# `ref_pat_eat_one_layer_2024_structural`
+
+The tracking issue for this feature is: [#123076]
+
+[#123076]: https://github.com/rust-lang/rust/issues/123076
+
+---
+
+This feature is incomplete and not yet intended for general use.
+
+This implements experimental match ergonomics under consideration for inclusion in Rust.
+- On Edition 2024, this corresponds to the typing rules found [here].
+- On older Editions, the current behavior is unspecified.
+
+For alternative experimental match ergonomics, see the feature
+[`ref_pat_eat_one_layer_2024`](./ref-pat-eat-one-layer-2024.md).
+
+[here]: https://nadrieril.github.io/typing-rust-patterns/?compare=false&opts1=AQEBAgEBAQEBAgIAAAAAAAAAAAAAAAA%3D&mode=rules&do_cmp=false&ty_d=3

src/doc/unstable-book/src/language-features/ref-pat-eat-one-layer-2024.md

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+# `ref_pat_eat_one_layer_2024`
+
+The tracking issue for this feature is: [#123076]
+
+[#123076]: https://github.com/rust-lang/rust/issues/123076
+
+---
+
+This feature is incomplete and not yet intended for general use.
+
+This implements experimental match ergonomics under consideration for inclusion in Rust.
+- On Edition 2024, this corresponds to the typing rules found [here].
+- On older Editions, the current behavior is unspecified.
+
+For alternative experimental match ergonomics, see the feature
+[`ref_pat_eat_one_layer_2024_structural`](./ref-pat-eat-one-layer-2024-structural.md).
+
+[here]: https://nadrieril.github.io/typing-rust-patterns/?compare=false&opts1=AQEBAAEBAQABAgIAAQEBAAEBAAABAAA%3D&mode=rules&do_cmp=false