Add the unshipped-backends generated section

A backend directory holding a NOTES.md and no .pte was exported and not
published. 31 such notes existed across the estate with nothing in the cards
pointing at them.

MODEL_CARD.md

@@ -1,7 +1,7 @@
 # Model card standard
 
 Every `software-mansion/react-native-executorch-*` repository carries a
-`README.md` in this shape. Four sections are **generated** from the repository
+`README.md` in this shape. Five sections are **generated** from the repository
 tree and its `config.json` files, so they cannot drift from what is published.
 Anything a model needs beyond them is a **free section**, written by hand and
 kept as-is.
@@ -77,7 +77,22 @@ which tokens are quantized, so the card does not repeat that partition. Nor
 does it name a default, because the library resolves one per platform and a
 second answer here would only compete with it.
 
-### 3. Repository structure
+### 3. Unshipped backends
+
+Omitted unless the repository has one. A backend directory carrying a
+`NOTES.md` and no `.pte` was exported and not published; the note records what
+went wrong, and the card links it rather than repeating it.
+
+```markdown
+## Unshipped backends
+
+A backend directory carrying a `NOTES.md` and no `.pte` was exported and
+not published. Its note records what went wrong.
+
+- `coreml` ([why](https://huggingface.co/software-mansion/<repo>/blob/main/coreml/NOTES.md))
+```
+
+### 4. Repository structure
 
 Every tracked file except `README.md` and dotfiles, with its size, so a reader
 can see the download cost before fetching.
@@ -95,7 +110,7 @@ xnnpack/model_xnnpack_fp32.pte     31.1 MB
 
 Sizes render as `B`, `kB`, `MB` or `GB`.
 
-### 4. Compatibility
+### 5. Compatibility
 
 Fixed text, parameterised only by the ExecuTorch runtime version. It states
 the runtime the files are **published for**, and deliberately does not claim