GH-46002: [CI][Release] Define an official Arrow C++ support policy

docs/source/developers/cpp/support.rst

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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
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+.. 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.
+
+.. _support:
+
+==============
+Support Policy
+==============
+
+Official Support Policy for Apache Arrow C++
+============================================
+
+The Apache Arrow C++ implementation supports various build systems, operating systems,
+and architectures. This has generated a growing ecosystem of Continuous Integration (CI)
+jobs to maintain in order to ensure the matrix of supported platforms is properly tested.
+
+This page documents how a platform (operation system, architecture or build system)
+is supported in the Arrow C++ implementation along with a list of platforms that
+are currently supported for the Arrow C++ implementation.
+
+Support tiers
+=============
+
+Platform support is broken down into tiers. Each tier comes with different requirements
+which lead to different promises being made about support.
+
+To be promoted to a tier, committer support is required and is expected to be driven by consensus.
+A vote may be called if there is no agreement about the promotion but general consensus
+on the GitHub issue is sufficient.
+
+Demotion to a lower tier occurs when a CI job is unmaintained for an extended period of time or the
+platform is going to be EOLed.
+
+If the job is unmaintained a discussion on the mailing list and if necessary a vote
+should be called in order to remove the platform from its current tier.
+
+When a Platform is EOLed, it should be demoted to Tier 3 immediately unless there is
+a strong reason to keep it supported.
+
+Tier 1
+------
+    - Is tested on CI on PRs or via periodic daily runs.
+    - Changes which would break the main branch are not allowed to be merged; any breakage should be fixed or reverted immediately.
+    - CI failures block releases.
+    - All committers are responsible to keep these CI jobs working.
+
+Tier 2
+------
+    - Is tested on CI via periodic daily runs and can be tested on Pull Requests via optional jobs.
+    - The CI jobs are prefixed with ``Tier 2``.
+    - Are maintained as a best-effort basis.
+    - CI failures do not block releases.
+    - At least one committer should look over the CI jobs and be responsible to keep these platforms working.
+
+Tier 3
+------
+    - Are not tested on CI.
+    - Unsupported though we might want to accept patches for them depending on the maintenance overhead.
+
+
+.. list-table::
+   :header-rows: 1
+
+   * - Platform constraint
+     - Tier