minor #58355 Add PR template and auto-close PR on subtree split repositories (GromNaN)

This PR was squashed before being merged into the 5.4 branch.

Discussion
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Add PR template and auto-close PR on subtree split repositories

| Q             | A
| ------------- | ---
| Branch?       | 5.4
| Bug fix?      | no
| New feature?  | no
| Deprecations? | no
| Issues        | Fix #57267
| License       | MIT

Extract the Pull Request Template from #54653.

~The GitHub action did not work as said by `@nicolas`-grekas symfony/symfony#57267 (comment) Replaced by https://github.com/superbrothers/close-pull-request.

Commits
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d90fa7ab08f Add PR template and auto-close PR on subtree split repositories

Author: nicolas-grekas

.gitattributes

@@ -1,4 +1,3 @@
 /Tests export-ignore
 /phpunit.xml.dist export-ignore
-/.gitattributes export-ignore
-/.gitignore export-ignore
+/.git* export-ignore

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,8 @@
+Please do not submit any Pull Requests here. They will be closed.
+---
+
+Please submit your PR here instead:
+https://github.com/symfony/symfony
+
+This repository is what we call a "subtree split": a read-only subset of that main repository.
+We're looking forward to your PR there!

.github/workflows/close-pull-request.yml

@@ -0,0 +1,20 @@
+name: Close Pull Request
+
+on:
+  pull_request_target:
+    types: [opened]
+
+jobs:
+  run:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: superbrothers/close-pull-request@v3
+      with:
+        comment: |
+          Thanks for your Pull Request! We love contributions.
+
+          However, you should instead open your PR on the main repository:
+          https://github.com/symfony/symfony
+
+          This repository is what we call a "subtree split": a read-only subset of that main repository.
+          We're looking forward to your PR there!