Split tutorial across multiple pages

.gitignore

@@ -4,4 +4,5 @@ dist
 docs/.ipynb_checkpoints
 docs/_build
 docs/_autosummary
+docs/my-data.zarr
 __pycache__

docs/aggregation.ipynb

@@ -0,0 +1,59 @@
+{
+ "cells": [
+  {
+   "cell_type": "markdown",
+   "id": "aaf2f2cb",
+   "metadata": {},
+   "source": [
+    "# Aggregation\n",
+    "\n",
+    "Xarray-Beam can perfrom distributed data aggregation in the \"map-reduce\" model. This currently only includes `Mean`, but we would welcome other contributions."
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "id": "4b77b953",
+   "metadata": {},
+   "source": [
+    "Note that {py:class}`Mean.Globally <xarray_beam.Mean.Globally>` and {py:class}`Mean.PerKey <xarray_beam.Mean.PerKey>` are modelled off of [`beam.Mean`](https://beam.apache.org/documentation/transforms/python/aggregation/mean/) rather than {py:meth}`xarray.Dataset.mean`: they compute averages over sequences of `xarray.Dataset` objects, rather than calculating an average over a dimension."
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "id": "37193539",
+   "metadata": {},
+   "source": [
+    "If you want to average over dimension, you'll currently need to do that aggregation yourself, e.g., by averaging inside each chunk before combining the data."
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "id": "c9db1627",
+   "metadata": {},
+   "source": [
+    "TODO(shoyer): add code examples! For now, look at [ERA5 climatology](https://github.com/google/xarray-beam/blob/main/examples/era5_climatology.py)"
+   ]
+  }
+ ],
+ "metadata": {
+  "kernelspec": {
+   "display_name": "Python 3",
+   "language": "python",
+   "name": "python3"
+  },
+  "language_info": {
+   "codemirror_mode": {
+    "name": "ipython",
+    "version": 3
+   },
+   "file_extension": ".py",
+   "mimetype": "text/x-python",
+   "name": "python",
+   "nbconvert_exporter": "python",
+   "pygments_lexer": "ipython3",
+   "version": "3.9.6"
+  }
+ },
+ "nbformat": 4,
+ "nbformat_minor": 5
+}