docs: enable SPI documentation rendering via DocC (#125)

## What's Changed

Swift Package Index automatically hosts rendered and browse-able
documentation when DocC is used.

This PR adds an overview of the API surface of this library (a Markdown
index file) that guides the documentation generator. It can be browsed
in Xcode, too, if you want a preview:

<img width="3608" height="2298" alt="2026-01-19 07-38-08 Xcode - Arrow
Documentation@2x"
src="https://github.com/user-attachments/assets/3c53a5df-67f1-4597-bad6-42c138af2a29"
/>

Enhancements would include more tutorialized content on how to use this,
but the project website exists for this for now.

Closes #128.

---------

Co-authored-by: Sutou Kouhei <kou@cozmixng.org>

Author: DivineDominion

.spi.yml

@@ -0,0 +1,21 @@
+# Licensed to the Apache Software Foundation (ASF) under one
+# or more contributor license agreements.  See the NOTICE file
+# distributed with this work for additional information
+# regarding copyright ownership.  The ASF licenses this file
+# to you under the Apache License, Version 2.0 (the
+# "License"); you may not use this file except in compliance
+# with the License.  You may obtain a copy of the License at
+#
+#   http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing,
+# software distributed under the License is distributed on an
+# "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+# KIND, either express or implied.  See the License for the
+# specific language governing permissions and limitations
+# under the License.
+
+version: 1
+builder:
+  configs:
+    - documentation_targets: [Arrow]

README.md

@@ -50,7 +50,7 @@ Then add `Arrow` to your target's dependencies:
 
 ## Getting Started
 
-TODO: We should refer auto generated documentation on Swift Package Index after we register this package to Swift Package Index.
+For API documentation and usage examples, see the [documentation on Swift Package Index](https://swiftpackageindex.com/apache/arrow-swift/documentation).
 
 ## Getting involved
 

Sources/Arrow/Arrow.docc/Arrow.md

@@ -0,0 +1,68 @@
+<!--
+  Licensed to the Apache Software Foundation (ASF) under one
+  or more contributor license agreements.  See the NOTICE file
+  distributed with this work for additional information
+  regarding copyright ownership.  The ASF licenses this file
+  to you under the Apache License, Version 2.0 (the
+  "License"); you may not use this file except in compliance
+  with the License.  You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+  Unless required by applicable law or agreed to in writing,
+  software distributed under the License is distributed on an
+  "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+  KIND, either express or implied.  See the License for the
+  specific language governing permissions and limitations
+  under the License.
+-->
+
+# ``Arrow``
+
+Apache Arrow: Columnar format for fast data interchange and in-memory analytics.
+
+## Overview
+
+Arrow provides a universal columnar data format optimized for efficient analytic operations. This Swift implementation enables you to:
+
+- Read and write Arrow IPC file and streaming formats
+- Build columnar data structures with typed arrays
+- Work with record batches and tables
+- Encode and decode Swift types to Arrow format
+- Interoperate with other Arrow implementations via the C Data Interface
+
+## Topics
+
+### Reading and Writing Data
+
+- ``ArrowReader``
+- ``ArrowWriter``
+
+### Core Data Structures
+
+- ``ArrowTable``
+- ``RecordBatch``
+- ``ArrowColumn``
+- ``ChunkedArray``
+
+### Arrays
+
+- ``ArrowArray``
+- ``ArrowArrayBuilder``
+- ``ArrowArrayHolder``
+
+### Schema and Types
+
+- ``ArrowSchema``
+- ``ArrowField``
+- ``ArrowType``
+
+### Encoding and Decoding
+
+- ``ArrowEncoder``
+- ``ArrowDecoder``
+
+### C Data Interface
+
+- ``ArrowCExporter``
+- ``ArrowCImporter``