GH-50115: [Dev] Adjust GitHub Copilot configuration for preliminary reviews (#50117)

### Rationale for this change

We tried auto GitHub Copilot reviews recently and discussed our experience for them:
https://lists.apache.org/thread/thq0dz19shxbrjypb81q5ltx8h0w54ob

We want to tune auto GitHub Copilot reviews:
* We don't need auto GitHub Copilot reviews for draft PR: https://lists.apache.org/thread/y7yc4yg9n4mdqd1y00w7s498y8m6yold
* GitHub Copilot review is too verbose: #50023 (comment)
* We need to document the expectation for contributors responding to GitHub Copilot reviews: https://lists.apache.org/thread/wvhdqqtq2ndxxjnqwnj39xfcys21tpc4

### What changes are included in this PR?

* Disable auto GitHub Copilot reviews for draft PR
* Add code review instructions for GitHub Copilot
  * Base text is generated by Gemini 3 Flash Preview and I adjusted it

### Are these changes tested?

No. We need to merge this to test this.

### Are there any user-facing changes?

No.
* GitHub Issue: #50115

Lead-authored-by: Sutou Kouhei <kou@clear-code.com>
Co-authored-by: Raúl Cumplido <raulcumplido@gmail.com>
Co-authored-by: Andrew Lamb <andrew@nerdnetworks.org>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>

Author: 3 people

.asf.yaml

@@ -41,7 +41,7 @@ github:
 
   copilot_code_review:
     enabled: true
-    review_drafts: true
+    review_drafts: false
     review_on_push: true
 
 notifications:

.github/copilot-instructions.md

@@ -0,0 +1,21 @@
+# Instructions
+
+## Code review
+
+You are a pragmatic senior developer. When reviewing pull requests,
+follow these rules to avoid noise and redundancy:
+
+- Be concise: Keep comments brief and to the point. Avoid
+  conversational filler or praising the code unless it's exceptional.
+- High-impact only: Focus on logic errors, security vulnerabilities,
+  performance bottlenecks, and breaking changes.
+- Skip the Obvious: Do not describe what the code is doing. Assume the
+  reader understands the code.
+- Ignore trivialities: Do not comment on minor style issues or things
+  that an automated linter should catch.
+- Single comment per issue: If the same pattern occurs multiple times,
+  mention it once and suggest a global fix instead of commenting on
+  every line.
+- First-time contributors: For users new to this repository,
+  explicitly instruct them to "Please check and address all review
+  comments in this PR."

dev/release/rat_exclude_files.txt

@@ -101,4 +101,5 @@ r/tools/nixlibs-allowlist.txt
 .gitattributes
 ruby/red-arrow/.yardopts
 ruby/red-arrow-format/lib/arrow-format/org/*
+.github/copilot-instructions.md
 .github/pull_request_template.md

docs/source/developers/overview.rst

@@ -104,6 +104,11 @@ When contributing a patch, use this list as a checklist of Apache Arrow workflow
   merged, this will be retained in the extended commit message.
 * Make sure that your code **passes the unit tests**. You can find instructions how
   to run the unit tests for each Arrow component in its respective README file.
+* In order to improve our code and your review experience, we have
+  enabled GitHub Copilot reviews to automate the first pass. Reviewers
+  and contributors are encouraged to address valid suggestions; you
+  don't need to respond to every comment, but please don't ignore them
+  either.
 
 Core developers and others with a stake in the part of the project your change
 affects will review, request changes, and hopefully indicate their approval