add missing index file in docs

docs/api-reference/index.md

@@ -0,0 +1,32 @@
+# API Reference
+
+Anything documented in the API reference is intended to work consistently among
+supported backends.
+
+For example:
+```python
+import narwhals as nw
+
+nw: DataFrame
+nw.with_columns(
+    a_mean = nw.col('a').mean(),
+    a_std = nw.col('a').std(),
+)
+```
+is supported, as `DataFrame.with_columns`, `narwhals.col`, `Expr.mean`, and `Expr.std` are
+all documented in the API reference.
+
+However,
+```python
+import narwhals as nw
+
+nw: DataFrame
+nw.with_columns(
+    a_ewm_mean = nw.col('a').ewm_mean(alpha=.7),
+)
+```
+is not - `Expr.ewm_mean` only appears in the Polars API reference, but not in the Narwhals
+one.
+
+In general, you should expect any fundamental dataframe operation to be supported - if
+one that you need is not, please do open a feature request!