docs: rename aggregations.rst demo df to orders_df to avoid clobbering state

The "Building per-group arrays" block added in the previous commit
reassigned `df` and `ctx` mid-page, which then broke the
Grouping Sets examples further down that share the Pokemon `df`
binding (`col_type_1` etc. no longer resolved). Rename the demo
DataFrame to `orders_df` and drop the redundant `ctx = SessionContext()`
so the shared state from the top of the page stays intact.

Verified with `sphinx-build -W --keep-going` against the full docs
tree.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: timsaucer

docs/source/user-guide/common-operations/aggregations.rst

@@ -179,18 +179,15 @@ total:
 
 .. ipython:: python
 
-    from datafusion import SessionContext, col, lit, functions as f
-
-    ctx = SessionContext()
-    df = ctx.from_pydict(
+    orders_df = ctx.from_pydict(
         {
             "order_id": [1, 1, 1, 2, 2, 3],
             "supplier_id": [100, 101, 102, 200, 201, 300],
             "failed":      [False, True, False, False, False, True],
         },
     )
 
-    grouped = df.aggregate(
+    grouped = orders_df.aggregate(
         [col("order_id")],
         [
             f.array_agg(col("supplier_id"), distinct=True).alias("all_suppliers"),