gh-39024: Automated the generation of changelogs in text format
    
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Associated with [#480 on website
repo](sagemath/website#480)

This pull request defines a workflow which is executed when any release
is published.
It in turn triggers the execution of `Generate Changelog` workflow in
[website repo](https://github.com/sagemath/website) and the changelog is
generated in .txt format if the current release was a stable release.

### 📝 Checklist

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- [x] The title is concise and informative.
- [x] The description explains in detail what this PR is about.
- [x] I have linked a relevant issue or discussion.
- [ ] I have created tests covering the changes.
- [ ] I have updated the documentation and checked the documentation
preview.

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URL: #39024
Reported by: Soham Rane
Reviewer(s): Kwankyu Lee, Soham Rane

Author: Release Manager

.github/workflows/changelog_trigger.yml

@@ -0,0 +1,23 @@
+name: Trigger Changelog Generation
+
+on:
+  release:
+    types: [published]
+
+jobs:
+  trigger-website-repo-workflow:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Trigger Generate Changelog Workflow in website repo
+        if: "!github.event.release.prerelease"
+        env:
+          GITHUB_PAT: ${{ secrets.WEBSITE_ACCESS_TOKEN }}
+          RELEASE_TAG: ${{ github.event.release.tag_name }}
+        run: |
+          curl -L \
+            -X POST \
+            -H "Accept: application/vnd.github+json" \
+            -H "Authorization: Bearer $GITHUB_PAT" \
+            -H "X-GitHub-Api-Version: 2022-11-28" \
+            https://api.github.com/repos/sagemath/website/actions/workflows/generate_changelog.yml/dispatches \
+            -d '{"ref":"master","inputs":{"release_tag":"'"$RELEASE_TAG"'"}}'