docs: Add quick guide for downstream builds

site/content/in-dev/unreleased/downstream-build.md

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+---
+#
+# 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.
+#
+title: Building a Custom Server Based on Polaris
+type: docs
+weight: 1005
+---
+
+This page highlights a few keys points for building custom servers based on Apache Polaris.
+
+Polaris produces several jars. These jars may be used to build custom catalog server 
+according to the terms of the license included in Polaris distributions.
+
+## General Principles
+
+* Create a module in your project (Maven, Gradle or any other build tool) for a
+[Quarkus Application](https://code.quarkus.io/).
+* Include `polaris-runtime-service` as a runtime or compile-time dependency.
+* Include / exclude other dependencies according to the project's needs (e.g., an alternative JDBC driver).
+* Do not include `polaris-runtime-defaults` or `polaris-server` as dependencies.
+* Provide your own `application.properties` file for your Quarkus build.
+* Define your own Quarkus Application Name and Version (do not use "Apache Polaris" as the application name).
+
+## Background
+
+The `polaris-server` module is intended to be a particular way to build a Polaris server. It includes
+dependencies that are considered essential for the Apache Polaris distribution. This is not necessarily
+the case for downstream builds, which may want to exclude some features and include others (perhaps
+custom implementations of some Polaris SPI interfaces).
+
+The `polaris-runtime-defaults` module is intended to hold `application.properties` configured for the 
+default Apache Polaris distribution. While it may be tempting to reuse it in a downstream build and
+add overrides for some properties, practice shows that in this case it becomes very hard to predict
+the effects of property customizations in the downstream build. While it may be relevant to Gradle builds
+only, it is much more clear to have the full set of properties in a custom `application.properties` file
+without any other `application.properties` files in transitive dependencies.
+
+The `polaris-runtime-service` module contains standard service endpoint implementations and is intended
+to be reused. Custom [CDI](https://quarkus.io/guides/cdi-reference) beans may be used in downstream
+projects to override / enhance beans provided by `polaris-runtime-service`.