[Doc] Convert docs to use colon fences (#12471)

Signed-off-by: Harry Mellor <19981378+hmellor@users.noreply.github.com>

docs/source/contributing/model/tests.md

@@ -14,14 +14,14 @@ Without them, the CI for your PR will fail.
 Include an example HuggingFace repository for your model in <gh-file:tests/models/registry.py>.
 This enables a unit test that loads dummy weights to ensure that the model can be initialized in vLLM.
 
-```{important}
+:::{important}
 The list of models in each section should be maintained in alphabetical order.
-```
+:::
 
-```{tip}
+:::{tip}
 If your model requires a development version of HF Transformers, you can set
 `min_transformers_version` to skip the test in CI until the model is released.
-```
+:::
 
 ## Optional Tests