docs: improve landing page (#1392)

Fixes #1385.

docs/source/csharp/index.rst

@@ -0,0 +1,26 @@
+.. 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.
+
+=======
+C#/.NET
+=======
+
+The ADBC C# libraries are under development, supporting:
+
+- A native driver to Google BigQuery
+- A way to import/export native (C/C++) drivers
+- Bindings to the Snowflake driver

docs/source/faq.rst

@@ -30,7 +30,7 @@ ADBC is:
   For example, result sets of queries in ADBC are all returned as
   streams of Arrow data, not row-by-row.
 - A set of implementations of that API in different languages (C/C++,
-  Go, Java, Python, and Ruby) that target different databases
+  C#/.NET, Go, Java, Python, and Ruby) that target different databases
   (e.g. PostgreSQL, SQLite, any database supporting Flight SQL).
 
 Why not just use JDBC/ODBC?

docs/source/index.rst

@@ -40,10 +40,12 @@ Apache Arrow ADBC
          **Cross-language**, **Arrow-native** database access.
 
 ADBC is a set of APIs and libraries for Arrow-native access to databases.
-Execute SQL and Substrait queries, query database catalogs, and more, all
+Execute SQL and Substrait_ queries, query database catalogs, and more, all
 using Arrow data to eliminate unnecessary data copies, speed up access, and
 make it more convenient to build analytical applications.
 
+.. _Substrait: https://substrait.io/
+
 .. div::
 
    .. grid::
@@ -160,10 +162,14 @@ Why ADBC?
    :gutter: 1
 
    .. grid-item-card:: Arrow-native
+      :link: https://arrow.apache.org/
 
       Execute queries and get back results in Arrow format, eliminating extra
       data copies for Arrow-native backends.
 
+      +++
+      Learn about Apache Arrow
+
    .. grid-item-card:: Backend-agnostic
       :link: driver/status
       :link-type: doc
@@ -202,6 +208,7 @@ Why ADBC?
    :hidden:
 
    C/C++ <cpp/index>
+   C#/.NET <csharp/index>
    Go <https://pkg.go.dev/github.com/apache/arrow-adbc/go/adbc>
    Java <java/index>
    Python <python/index>